I think we all buy fish with ich every day, knowingly or not. If the question is, would I buy a fish that is showing symptoms of being infected with ich, such as white spots, flashing and rapid breathing, then the answer would be yes.
Since finally seeing the light and realizing the great benefit of Hypo QT of every fish, I have no qualms about buying a fish in distress. I wouldn't even require a discount on the infected fish since most LFS's give a 7-14 day guarantee.
I think as responsible reef/fishkeepers, we shouldn't just try to cherry pick the best fish possible. Meaning, if you have the knowledge to properly QT and heal a fish infected with ick then we should try do so since we know they should be QT'd anyway. For those who don't know, believe or care to QT, let them pick the healthiest looking fish.
Wow, I bet I'll catch some flack for this point of view!