Now, I may disagree with Dmong on a great many things, but I have to agree with him on this one. A person buying a snake from pet store may be trying to buy something to breed themselves to create... who knows... there own hybrid. Perhaps, your hybrid cornsnake that looks pure was selected by them to breed to their own pueblacorn? Buyers don't always disclose their intentions to those they are buying from even if asked. They may be afraid that you might not approve of the hybrids they are going to create or simply think its none of your business etc.
Regardless, if your selling neutered snakes you should list them as neutered. I'd much rather by a snake that was not neutered though and simply be told this is not a pure corn, it may look pure, but its not. It is 1/4, 1/4, and 2/4 this. You give that person the ability to use that knowledge and yes, even make their own hybrid back to one of the parent species. Who knows, they may even be just as ethical as you and they may come up with some phenotypical expression you might have missed as you did not make that cross with that specimen? Even if it turns up a dead end for that person with that cross... it does not mean that they have to stop there. They can continue with their attempts until they do create something worthwhile.