05/30/2012 06:48 pm ET Updated May 30, 2012

Yard Sale Treasures Lurk Amid Junk, Antiques Appraiser Says

In the world of yard sales and antiques auctions, the thrill is in the hunt for hidden treasures. Take those old dentures from Grandpa, especially if you're the grandson of Winston Churchill's dentist. Value? $4,000.

The expert appraiser behind that hidden gem is Patrick van der Vorst, 41, who worked as a director at auction house Sotheby's for more than a decade in London. Three years ago, he started the website to bring auction-house expertise to the masses, in the United States and the United Kingdom.

Now, for a $9.99 fee, yard sale scavengers and flea market hounds can send in a photo of their find and within 24 hours get an expert opinion on the object's value. With yard sale season picking up, we called van der Vorst to get the insider's expertise on what is valuable -- and how to know if you're sitting on your own antique gold mine.

What's the craziest piece of "trash" submitted to your website that was actually worth a million dollars?
I wouldn't say a million [dollars], but we evaluated a chastity belt that the owners thought was an implement for a horse. We valued [it] for $5,000 … What looks like trash is probably 1970s furniture that people see as old-fashioned but is now sold as secondhand. But it's transgressing from secondhand pieces to collectors' items. That is where we see a lot of the surprises.

How can you tell if you've got cash in the attic?
Anything that is signed -- whether furniture or painting -- could be an indication of quality and therefore desirable. The other thing is condition. It needs to be in good condition to [sell] well. It's also following gut instinct about what might be desirable to someone else.

Is it better to be able to spot valuable antiques or understand the market overall?
It's combination of both. You have to understand the market because at the end of the day that is what an auction does: You place an item on the market, and the market decides what the final sales price is. Even if you know the whole history, it won't help you sell it.

What about selling tips?
If [you have] antiques, you are better in [a] live auction. If [you have] collectors' items, like baseball cards, then the market is online. If [it's a] painting, [go to] a dealer. eBay is more for collectable items like comics, toys and dolls, ceramics and not very rare items. If you have housewares like six crystal glasses, then do sell that on eBay.

You're a Londoner. What is the difference between how Americans and British people think about their money?
Americans are prepared to take more of risk to gain more. Europeans are quite conservative with their money. I can see that Americans can more easily take the plunge to buy art and hope it will go up in value in five years' time. Europeans are more cautious [about what] they buy and invest [in] and they want to check a few more boxes.

Are you saying Europeans are cheap?
[Laughs.] I wouldn't say cheap, but they want value.

What do you keep in your wallet?
There would be 150 [British] pounds and $100 because I was in New York last week and I have about 100 euros because I travel to the Continent a lot.