When artists agree to be represented by a gallery, they usually work out with the gallery owner -- formally or informally -- the terms of their understanding. One expectation, which may or may not be stated explicitly, is that the dealer will keep track of the artist's work even after it has been sold.
There are many reasons for and against making one's own paints. On the pro side, the cost of purchasing the components in artists' paints add up to considerably less money than the price of buying commercially available tubes, plus the fact that those who make their own usually buy in bulk, which increases the savings.
This fear is never far from the minds of collectors in the art multiples field: Do copies of an original image have any intrinsic value? (Does the print itself have value, or are buyers only paying for the expense of operating a printer?) Does limiting the number of prints in an edition actually increase that value?