You know I think I'm starting to change my approach to lfs's that treat their fish poorly. I'm starting to see myself as a small rescue shelter. If I find a fish that is in poor health at a fish store I'll consider buying (if I can) just to save the poor guy. Of course, I can't buy every sick fish I come across, but if I was planning to buy one anyway, I may take the chance. Of course, I realize that can get really expensive (so keep your receipt for a credit or exchange). I also understand that not every fish can be saved.
Of course, you don't want to introduce that fish to a healthy tank, so you have to quarantine. I treat my new fish as though their sick already. I immediately treat for ich (usually with higher water temp to start out with). I'm still trying to find a safe method to treat for other concerns, like internal parasites or fungus. But if you get a fish from the store with a condition you can already diagnose then you're already on the way to finding the right treatment.