Really general condition is all you need to look at. If the intake and exhaust are stock, the motor should be fine. Take a look at the chain and sprockets to see if they've been maintained properly. There are really no problems that pop up in the used bikes, your biggest concern should probably be if the bike has been laid down or wrecked.
Edit: in the case of the 09 with 4k miles for 2500, I'd really check if its been dropped or wrecked. I hardly see them for that low, so either something might be wrong with it or the guy just really needs to sell.