I don't have any personal experience but I have not heard very good things about used items from Cowboom on from other deal sites.
Whereas places like Amazon will say things like may have scuffs but 99% of the time the item looks brand new, with Cowboom I hear it's the opposite, things are usually quite beat up.
Like I said, just hearsay, I don't know this first hand but I was looking into buying some tablets from them before and the comments scared me off.