You need to stipulate what rights the buyer has at the time of sale. If it's a unique piece of art as opposed to something that can be replicated many times or as a limited edition once you sell it it's no longer yours to exercise any control over.
Originally Posted by jnanian
It's a bit like some of the early 20th C painters who struggled to survive and sold their paintings to live, later seeing them sell on later for very high prices and getting nothing.
So you need to make the highest possible quality copy before you sell it and retain the right to use the copy as you wish, perhaps stipulate the new owners of the image can't use it for commercial purposes. So essentially you want to retain the reproduction rights.