Well if I was in that same situation I would badger them into letting me have him to have him for free ...and feel better from poopy butt,,,, Don't take my advice as serious because I am very errr...I don't want to say careless because I can care for my rabbits... but I act on instinct and want to do what is best. Obviously the store isn't going to sell a sick animal so maybe you could work something out with them???When I worked at petco here in the states we HAd to bring them to the vet if ANY animal was sick. I don't know, how is anya and bugz doing? do you want a 3rd? It's very nice you are willing to help...are you looking for a trio? It sounds like that little fellow needs someone to save him and quick. If I was you, I would adopt him and do whatever I can to save him, but I am not you, and I don't know your situation. I know it can eat away at your heart. but when we buy from a petstore we don't solve the problem of homeless rabbits from a shelter looking for a new home. Its tough I know. But I know if I saw that I would take him home with me and try my hardest to make him well, petstore, craigslist or shelters or not.
Remember I basically got all 3 of my rabbits as random and didn't think clearly but this is how I feel when I read this. If I saw a rabbit in need from a petstore I would do what I could to help. It just sucks to know that if you do adopt him, that just opens up a new spot for them to buy more rabbits. It's just a vicious cycle that never stops. It's really up to you and how you feel.