My fiancee bought her last car from one of these places. It came with a limited warranty and they even let her take the car to an independent mechanic for a second opinion. That alone addresses many of the potential pitfalls associated with buying any used car. No hassle/no haggle usually means paying more but it was worth it to her to not spend hours in a dealership negotiating over price.

A few years ago, I entertained the possibility of selling my car to Carmax. The price they offered was $1500 below KBB wholesale. You don't get the best prices (buying or selling) but the salespeople really were no-pressure.