So if they aren't feeding in the store than just use the credit for another fish?
any other pointers for picking out a healthy one? they all looked pretty lazy to me.
It is hard to describe what a superficially healthy fish look and behave like in a short post. It mostly comes with experience. Sorry about your loss and don't despair. Keep trying.
"Lazy" described a vast majority of catfish, who are mostly nocturnal creatures.
You can have the LFS hold one for you until you make sure it feeds well and is in good health.
Surely, it'd be prudent to make sure your water didn't kill him. I'd just try my best to make I didn't kill him and focus on trying again.
Good luck, mate!