I prefer to buy only tank-bred fish. I don't think I have ever seen an endangered fish for sale at my lfs, the guys are pretty good at avoiding such fish. If I were to buy one, that would be advocating the capture of more of these fish from their wild environment, i.e., my money would be going to the person who is breaking the law and harvesting these fish illegally. So, no, I would not knowingly buy a fish that I knew was endangered.
I know that there are people out there who dedicate a portion of their tanks to keep fish that are threatened and/or extinct in the wild. Such as certain varieties of goodeids, whose habitat is being destroyed in Mexico. Instances such as these, it is a good thing to keep endangered fish. But just to have a cool, rare, threatened fish in your living room to show off to your buddies? Not good.