I've bought dozens of GPUs from eBay, see my collection:
http://www.sgidepot.co.uk/misc/gpushelves15.jpg
If you want to play it safe, only ever bid on items being sold by sellers that mark their
listings as, "Returns Accepted". Remember though that past whatever cover eBay/PayPal
offers, you normally won't have a warranty.
And of course, the usual tips: check a seller's feedback, focus on listings that have
good descriptions, lots of pics, usage info/history, doubly so those that include GPU-Z
validation/screenshots to confirm functionality.
Also, don't bid too early on a listing. If you can, bid the genuine maximum you'd be willing to pay
in the last few seconds. Early bidders often bid less than they'd be willing to pay, for some reason
thinking that somehow it will mean others are less likely to bid (amazing how many people bid only
the starting price of an item). I bid in the last few seconds, exploiting this behaviour, often the route
to a good bargain. I don't use sniping tools though, I just do it manually (fast net link helps of course).
Lastly, be careful of bidding too much for something. Always have in mind what various GPU models
cost to buy new from normal sources. Atm for example, I see lots of people bidding crazy high amounts
for used 256GB SSDs, when one can buy brand new ones for the same or less, eg. three times in the
first 2 weeks go January, people paid between 80 and 100 UKP total for a SanDisk Extreme II 240GB,
yet the SanDisk X300 256GB is available brand new for less than 80 (I bought one last week new for 72).
So, don't pay over the odds. Always use a genuine maximum bid and stick to it. If you don't win, never
mind, be patient, more will come along.
Ian.