Just my two cents: I don't know who is recommending purchasing used GPU's to you, but IMO buying a used GPU right now is a real mixed bag. At the moment there is a huge amount of availability of cheap mid range GPU's on the market. This is mostly as a consequence of the 'mining' craze that happened about 3 years ago. If you find a GPU that is pretty decent, lets say a RX480/580, GTX1060/70 and they are for very low prices, then it;s likely they will have been used for mining. This means the GPU would have been run pretty much 24/7, and as a result could have problems or a short life span due to overuse.
Be careful with your purchase. Make sure the seller is reputable. Ask for a history (although they could bluff anyway). You know the saying: If it's too good to be true, it probably isn't'.
With that said, depending on your needs some GPU's will be suitable.
As an example a GTX1060 new now costs about $200. If you saw one for about 150, then that's a fair price for a used GPU (one could argue either way about $10 here or there). If you find the same GPU for $100 you know there is something rotten in Denmark!
Just be careful.
I'd agree with DSzymborski, I always buy new. You have the warranty, returns policy and peace of mind.