Why?Please don't be too hard on the stores for not buying fish back.
They sell the fish and take our money. They don't always get it right and customers pay the price with fish that breed or kill or grow too big.
If a shop sells a fish to a customer, and the fish gets too big or reproduces like crazy, the shop should take the fish back if the customer doesn't want them.
We used to buy fish back from customers and never had an issue with new diseases being introduced into our tanks. The diseases are already in the shop and the importers and wholesaler's aquariums. If a customer had an oversized fish we couldn't take, we told them it's too big for our tanks. But we also suggested (or called) a couple of other shops and tried to find a home for that fish.
We didn't carry some fish because they got too big. eg: Black sharks and salmontail catfish. We didn't get them in because they get huge and customers always needed to get rid of theirs because they outgrew the tank. But most other fishes that didn't get bigger than 10-12 inches, we would take them and either add them to the display tanks or put them up for sale.
We also used to advise customers on how big certain fish got and whether they were agro mongrels that would kill everything they were kept with. We told customers about common livebearers breeding prolifically and that they would probably have a lot of young fish in the near future. We said if the fish were similar sized to the ones we sell in the shop, we would take them (assuming they were healthy and not deformed).
We got all our bristlenose catfish and most of our African Rift Lake cichlids and Central American cichlids from local breeders. They were usually bigger and better quality than the stuff coming from the importers/ wholesalers.
There's no reason for pet shops to refuse to take average aquarium fishes. I'm not talk tank busters like redtail catfish or black pacu. But if a shop sells those fish, they should accept those fish back if the owner no longer wants them or can't care for them any more.