I'd consider a few things before buying.
Find out if there is any warranty left. If none, make sure you give the phone a thorough once over. Nothing worse than getting a "deal" only to find out something like the home button doesn't work right and then either dealing with repair costs or living with the problem.
Also consider how many charge cycles are on the battery. No real way to check this without installing one of many apps, but if it's a launch day device it can easily have 400+ cycles on it already if it was owned previously by a heavy user. Nothing necessarily wrong with a well loved device except that the battery may be getting worn to a point that you're going to see notably diminished capacity.