Agree with the above two gentlemen. Disagree with Mr. @jonzoo. Sure you can find some amazing offers when buying used hardware, and even find a better card for the same money. However, I do not consider this a valid approach when only upgrade one part.
I do acknowledge that sometimes the "used-hardware" option is superior, but for me the risk-taking does not justify the, somewhat, small price-drop.
Buy new:
1) Get warranty.
2) You can be sure it works. If it does not, RMA is an option.
3) Its a new product. You are the first user.
The only time wherein I consider used parts, is essentially if I am building a budget PC from scratch, or if I see an amazing offer from a seller close to my location, so I can visit that person, and confirm that the hardware's functionality is satisfactory.