I always talk to potential buyers and stress what they have to have in order to successfully keep the geckos (I sell leopard and fat tail geckos). I usually have complete set-ups available so they can buy the whole deal from me. If they don't have, or are unwilling to provide the set-up (or unwilling to quarantine) I have a concrete reason not to sell to them. I have not yet (in 5 years) had to tell anyone an outright "no". A few times I'll admit I've sold to a person where I wasn't 100% sure, but I also couldn't put my finger on something specific so I took the risk. Other times, I haven't had to say "no" because as a consequence of our discussion, I've gotten them to come around to the idea that this is not a good idea. In those cases, it's usually because they don't have or won't provide something, or they're not so sure and I give them my card and say "if and when you're ready, let me know". A number of people (usually parents of small children) at shows have thanked me for not doing a hard sell. Being willing to discuss the situation and not sell any gecko to any person "no questions asked" eliminates most of the potential problem. On the other hand, I don't feel the need to grill everyone endlessly about how they're going to take care of the geckos and to obsess about how everyone is doing.
Aliza