Thats sort of a difficult topic.
It depends alot on the kind of use the card saw before getting sent back, what exactly was the issue (if any), and who does the refurbishing.
Not PC related, but Canon refurbishes their cameras and sells them directly, they are essentially new and are inspected better than new models. I trust them entirely.
I wouldnt trust amazon nearly as much.
Chances are, the card could have just been returned without being used much if at all.
Or, it could have been used for mining, returned, and a tech says "looks good" and sends it back out.
Personally, I wouldnt buy a refurb card from amazon. A manufacturer directly, sure.