You pretty much take a chance when you buy used or refurbished on places like ebay. I would first check out Newegg and Amazon for used/refurbished GTX 1050 Tis. They at least offer safer return policies. Lastly... if you do go to ebay and look for a seller with a near-100% rating, lots of sales, and returns accepted, you will probably do OK.
I've bought a ton off of ebay and while I've had a few disappointments, I've never really been burned. You just have to look for a reputable seller like I outlined above.