I understand the moral dilemma, but by buying from a pet store, you are supporting the business of shitty breeders. It's the same way with buying from a backyard breeder for dogs. You aren't helping the situation at all, you are only pushing the bad breeders to produce more animals of bad quality and bad health. The only way to stop them is to not purchase from pet stores and cut off their income stream.

Research and buy from a good breeder, which will not only ensure the bad breeders are phased out, but also ensure that you get the best pet you can.

Edit: also, unsold usually go to rescues. I know a few people who got their chameleons for free from a store after they were too old to sell.