its going to depend on the person. I mean some people won't trust hets even from BHB or NERD, some people are too trusting and you could sell a het *insert make believe morph here*
I think taking pictures of the locks, marking the eggs, taking pictures of snakes hatching out of the eggs that are marked, clutch pictures bout all you can do. after that its just going to depend on the person interested in the hets. you have no control over it as a seller.