If you believe the veritable hordes of anonymous sources and "insiders with knowledge of the situation" who have been whispering in the ears and inboxes of nearly every tech blogger on the planet, the iPhone 5 is scheduled for release in mid-to-late September. The generally-reliable folks at Apple blog iMore have pegged the specific release date as Friday, Sept. 21, a date that, if correct, not only represents National Miniature Golf Day, but also the day on which many Americans will attempt to sell their old iPhones online in order to make way for the shiny new one while grabbing some cash back in the process.

But if you are looking to sell your current iPhone, and you wait for the new iPhone to actually hit shelves, you're probably losing out on a good deal of money. Nothing illustrates the money you're leaving on the table by waiting like a good old-fashioned line graph, and Gazelle.com, an electronics website that buys old gadgets, has released just that: an informative chart that should make you think twice about holding on to your previous-generation iPhone for too long.

Here's Gazelle's graph, which tracks the average price the site paid users for an iPhone 4 in the weeks leading up to the release of the iPhone 4S; this year's price movements should mirror last year's, as Gazelle accepts iPhone 4S trade-ins in the weeks leading up to the "iPhone 5" announcement:

iphone 4 value gazelle

Now, there are several things that potential iPhone sellers should note here (and from other, similar charts).

First, the iPhone 4S was announced on Oct. 4: After that date, the price declined steadily, but not dramatically.

Second, the iPhone 4S was released on Oct. 14: After that date, the price declined steadily, but, again, not dramatically.

Third -- and most importantly for someone trying to maximize resale value -- the most drastic price dive occurred not on the announcement date, nor on the release date, but instead on the rather innocuous Sept. 21, 2011. Know why? That's the day that Wall Street Journal-affiliated blog All Things Digital reported that the next iPhone would be unveiled by Tim Cook on Oct. 4. Because AllThingsD is such an especially reputable source for Apple news, and because reporter John Paczkowski was able to give a definitive date as well as event details, prices for old iPhones began to plummet, losing $70 in trade-in value in about 10 days. From a peak of over $250 in the days leading up to the AllThingsD reveal, an iPhone 4 fetched just about $185 on the day Apple announced the iPhone 4S (and it didn't go on sale, remember, until Oct. 14, at which point it was down to about $165 in trade-in value).

That precipitous decline in trade-in prices for the iPhone 4S hasn't occurred quite yet, Gazelle's Chief Gadget Officer Anthony Scarsella said in a telephone interview: A 16GB AT&T iPhone 4S in good condition still fetches $277 on Gazelle, or more than enough to defray the initial cost of buying a 16GB AT&T iPhone 5 when it becomes available next month. Though well-respected news outlets, including Reuters, Bloomberg, The New York Times, and -- yes -- All Things Digital, have reported that Apple is expected to hold an event the week of Sept. 11, iPhone sellers are apparently waiting for a more concrete announcement, which could come any day now.

Scarsella assumes that the bottom will fall out of these inflated iPhone prices near the beginning of September, or about two weeks before Apple's iPhone event and three weeks before the actual release; that's in line with when prices started falling last year. Keep in mind, too, that on many gadget trade-in sites you can lock in your price for weeks: On Gazelle, an offer is valid for 30 days, while on NextWorth, you have 21 days to actually mail in your iPhone. That means you can claim the value of your phone and then hold on to it for a while before you actually say goodbye and put your old mobile in the mailbox. Any smart seller will lock in their price as soon as they can, because the amount of money that Gazelle, NextWorth, eBay and the Amazon marketplace will be willing to offer for an iPhone 4S will only be decreasing over the next several weeks.

Though you'll want to ship away your current iPhone as quickly as possible for maximum cash, there are obvious hurdles you'll have to overcome, too. First, we won't definitively know the iPhone event's date until Apple officially announces it, and once Apple does announce it, resale value will go down; that's a fair amount of guesswork, and a lot of faith you'll have to put on those anonymous sources and whispering insiders. Second, you can't know when the iPhone will be available at your local store: What if supplies are extremely constrained, and it's a month before the "iPhone 5" is in stock? And, third: How can you know if you'll even like the next iPhone before you've seen it or know what it's capable of?

And yet, if you really are serious about semi-blindly committing to the next iPhone while also reaping in cash-money from a re-sale of your old iPhone, you should start devising your game plan now. A few questions to ask: Do you have an old cellphone that you can pop your iPhone's SIM card into for a few weeks? How long can you go without a smartphone? When do you estimate you'd be able to actually obtain the new iPhone, given a release date of Sept. 21? The answer to those questions will largely determine just how soon you'll be able to sell off your old iPhone, and thus how much money you'll be able to get for it.

As Anthony Scarsella of Gazelle reiterated to me several times over the course of our phone call, the earlier you trade in your iPhone, the better. In other words: Sell it today, if you can, and if not today, then tomorrow. If you're going to be one of the tens of millions who upgrade to the iPhone 5 this September, why not see if you can turn a profit on your purchase?

For more, check out the Wall Street Journal on why now is the best time to sell your iPhone:

Also on HuffPost:

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  • The iPhone 5 Is Coming In A Month: How Big Will The Screen Be?

    At this point in the game, we're pretty sure that <a href="http://www.huffingtonpost.com/2012/08/03/iphone-5-announcement-ipad-mini-this-week-in-apple-rumors_n_1738536.html?utm_hp_ref=technology" target="_hplink">Apple will unveil its next iPhone on September 12</a>, or somewhere close. All that remains is to figure out what the heck this thing is going to look like. We think -- <em>think</em>! -- that the major difference between the so-called "iPhone 5" and iPhone 4S will be a larger screen: Rumors put the screen size of the iPhone 5 at 4.0-inches diagonally, up from 3.5-inches diagonally on the iPhone 4S. This week, <a href="http://9to5mac.com/2012/08/07/upcoming-ios-6-is-scalable-to-taller-640-x-1136-iphone-display-shows-possible-next-generation-device-user-interface/" target="_hplink">9to5Mac wunderkind Mark Gurman was toying around</a> with a pre-release of the iOS 6 operating system and found that it conveniently scales up to a taller iPhone display, with 5 rows of icons instead of 4. That would seem to be a sign that when iOS 6 ships, along with the new iPhone, it will fit fine on both the 3.5-inch display of the iPhone 4S AND the taller display of the iPhone 5. How big will that display be? Well, <a href="http://www.appleinsider.com/articles/12/08/08/apples_new_iphone_expected_to_be_18_thinner_than_iphone_4s.html" target="_hplink">according to a fresh leak from the Chinese Apple.pro</a>, it's still on track to be 4.0-inches. More interestingly, that report claims the next iPhone will be 18 percent thinner than the iPhone 4S, chiseling down the width of the phone to 7.6mm. That would be pretty, pretty thin, if it's true; alas, Apple.pro isn't the most reliable of sources. Still, <a href="http://www.usatoday.com/tech/news/story/2012-07-30/apple-samsung-smartphones/56592972/1" target="_hplink">after Peter Svensson of the Associated Press</a> called the iPhone 4S "small and chubby" compared to the top Android phones, we'd be very surprised if Apple didn't substantially thin its new iPhone. And now: Photos of a bunch of supposed iPhone 5 parts! Go go gadget porn!

  • Look At All These iPhone 5 Parts!

    Sorry, friends: We didn't get any tantalizing videos or snapshots of a fully constructed iPhone 5 this week. But we did get the next best thing: a random smattering of wiring, circuits, and plastic doodads that could be part of the next iPhone! At left, you can check 'em all out, breathe them all in, meditate on them, consider their implications. Probably the most exciting thing here is the front cover, which does hint at that taller screen we were just talking about. Otherwise: Unless you are a very special kind of person, with very eccentric tastes, there's not much here to arouse your interest. Also in the Apple rumor-sphere this week, a <a href="http://www.macrumors.com/2012/08/10/next-generation-iphone-battery-only-marginally-higher-capacity/" target="_hplink">supposed photo of the iPhone's battery leaked out</a>, and it appeared to be about the same size and capacity as the current iPhone's battery. <a href="http://www.bgr.com/2012/08/10/iphone-5-battery-leak-lte-connectivity-with-1440-mah/" target="_hplink">Cue Apple blogger hyperventilation</a>: Because the next iPhone is all but certain to come with 4G LTE -- which sucks battery life faster than [FAMILY PUBLICATION] -- a similar-sized battery can only mean that the iPhone 5 will have power drain issues! Meanwhile, back on earth, we're going to wait until the new iPhone is released until we fret about its battery life. Moving along.

  • Evidence Of Smaller Retina MacBook Pro Spied Again

    Right now, if you want to enjoy some high-definition Retina MacBook goodness, you have to buy a 15-inch model. Over the past month, however, we've been hearing whispers that Apple could be adding to the family with a 13-inch Retina MacBook: Apple oracle Ming-Chi Kuo of KGI Securities said as much, and a device that looked a lot like a 13-inch Retina MacBook showed up in the Geekbench results page back in July. Now, just like the Spice Girls, that Retina-quality 13-inch MacBook Pro is BACK, once again appearing in the Geekbench database. <a href="http://www.macrumors.com/2012/08/10/13-inch-retina-macbook-pro-once-again-shows-up-in-benchmarks/" target="_hplink">MacRumors has all the (very, very) nerdy details</a>, for those who are in to feeds and speeds. For our purposes, however, let's just note that a 13-inch MacBook Pro with Retina display is looking more likely and could be on track for an October release, as previously suggested.

  • iPad Mini Mockups Abound!

    No new iPad Mini "news" this week -- we're still at the point of debating when this bad boy is going to come out (in September, along with the iPhone 5; or in November, just in time for Christmas; or perhaps in March 2013, along with the <strike>iPad 4</strike> new iPad). We do, however, have the next best thing: Mockups! Here's one from the designers at InventHelp (<a href="https://www.facebook.com/inventhelp" target="_hplink">find them on Facebook here</a>), from a Gizmodo article that was accurately titled "This Is How the iPad Mini Might Look Next to the iPad." And, yes, this is how an iPad Mini might look next to the iPad! For more photos of how the iPad Mini might look like next to the iPad, I would highly encourage you to visit the <a href="http://gizmodo.com/5932809/this-is-how-the-ipad-mini-will-look-next-to-the-ipad" target="_hplink">Gizmodo post titled "This Is How the iPad Mini Might Look Next to the iPad."</a> (There's also more iPad Mini mockups, also from InventHelp, <a href="http://www.zdnet.com/steve-jobs-was-receptive-to-7-inch-ipad-7000002209/" target="_hplink">at ZDNet this week</a>). And that, ladies and gentlemen, is the end of this week's edition of This Week In Apple Rumors. Make sure you check back next week, when we'll detail not only how an iPad Mini might look next to an iPad, but also how an antelope might look next to a peanut better and jelly sandwich (SLIDESHOW)(HORROR). And remember, if you can't wait until next week, you can get up-to-the-minute Apple rumors by <a href="http://twitter.com/gilbertjasono" target="_hplink">following me on Twitter right here</a>.