I buy used books at amazon, and I look for the cheapest copy from a vendor who's got a rating of 98% or better. I got that stringent after getting annoyingly burnt by indoo.com, which had a 95% rating. I'm not sure exactly what a 95% rating means--on the one hand, people who are pissed off are perhaps more likely to send in a rating than those who are pleased, and on the other, amazon makes it a little harder to send in a negative rating--iirc, just one of those "are you sure?" screens, but it's still something.
Anyway, I've been noticing that well-rated vendors are getting rarer at the low end of the price range. I assume reliable vendors are getting a more accurate idea of the value of their time.