My solution to avoiding uneducated or neglectful buyers is pretty straightforward; don’t sell or give away any birds in the first place. A while ago I gave some bantam roosters to a friend to temporarily care for. All those roosters ended up dead, eaten by predators. If I decide to breed my chickens again, I’ll only be hatching a limited amount of eggs and will have a plan for all chicks that hatch.
Generally, it’s a good idea to have a backup plan even if you plan on selling your birds, because, as you know, sometimes that doesn’t work out. You may want to also stop selling chicks. Chicks are usually less expensive, more likely to be bought on an impulse, and have certain needs that adult birds don’t. Also, someone who buys chicks may find that they don’t have adequate housing for them when the chicks grow up. So, a lot of your problems can probably be avoided by only selling adult birds. Or, you could even sell fertilized eggs instead. Selling eggs at least guarantees that someone has an incubator or already has birds that are broody, which means they probably at least have some experience, have done some research, and are probably better prepared for chicks once they hatch.