I've shopped at GameStop but never bought anything that has been used.
My daughter had a pink DS (we searched high and low one yr for it) and she loved it.
My brother's teens love GameStop and buy used games all the time. Actually, they are getting GameStop gift cards this year.
I will say that I had a good experience at the GameStop in Frisco. Last year, I bought the Band Hero that had the drums, and works. When daughter set it up, the drums did not work. I sent the symbol to the manufacturer as directed by GameStop and when I got the replacement, it did not work either. The manager at GameStop ended up giving me a complete new set and took the other one back, returned it to the manufacturer as defective.
Technically, he is not supposed to take things back. He said he did it because we had to deal with a non working holiday set for 2 months. He didn't have to go above and beyond like he did. Most places would never have that type of customer service.
Good Luck