Luck and timing plays a role.
Yeah, I've bought two 5870's from there. The first one was perfect from the get-go. The second one I noticed idled at over 45oC, probably why the guy sold it. Luckily it just needed a better thermal paste job... i'd be hard pressed to recall a time where i'd seen so much thermal paste caking all over the place
(and my price point on 5870's is $165 now, many sold in the $160-$165 range on ebay)