You are correct. My local petco won't buy fish, but they will often accept healthy fish from people who can't or won't keep them. They try to keep these separated from their regular stock whenever possible, but still in display tanks. These fish are then 'adopted' back out. Over the weekend someone brought in 6 large beautiful cichlids and left them at my local petco while I was there on a sales call. The weekend before someone else dumped a horrible mishmash of community fish on the same store. They can't give credit for turn-ins, unless you are actually returning a fish according to store policy, but petco is pretty good about helping people rehome their pets - that goes for reptiles, rodents, and birds, too!I don't think chains like Petco and Petsmart take fish. I am sure there is exceptions.
There's no one answer for this question. Depends on the store. Depends on why the fish are being returned. Depends on the employees working that day. Depends on the policy the store has.