This is not how possible hets work. You have a snake that is 50% possible het. It either is, or it isn't. If you don't prove it out and it DOES carry the gene, the babies from that snake will also be 50% het, only you won't know, because you never proved out the parent snake's genes. When selling you can, and possibly should, let people know that the parent is 50% possible het whatever but hasn't been proved.