The only fish I've ever taken to a shop was a large cantankerous sucking loach. I avoid them at all costs now and get annoyed when I see them for sale as I think they're completely wrong for a community tank (pet hate - can you tell?!)
If any of you have read the recent PFK about fish longevity, it's sometimes surprising that fish can live as long as they do. It's not unusual to have neons etc living for nearly 10 years. If you didn't swap them, you'd either have alot of tanks, or not be able to keep that much. In my community tank I've got 1 lemon tetra, 1 harlequin, 1 cardinal, 1 variatus platy and 1 ruby barb that are all the last remaining ones from their respective shoals that have existed over time. I've also got one elderly moonlight gourami. I've got quite attached to them over time. I know they're fish, but I feel responsibility towards them and wouldn't want them ending up with a noob and used for cycling his tank!
Maybe we should start a fish swap/donate scheme?
Anyone want two angelfish with ASBOs?