if you still have a transferable(?) guarantee that will be good for frame/bb/headset;
as a purchaser : cleanliness of drive train + rims/tyres. (which you can all show in photos)
(would be interesting to know how depreciation compares to cars 20/30/50 1st/2nd/3rd)
Uhm, it has lifetime warranty, still have original receipt and all is in tip top shape and clean as hell lol