I have used ABC Gift Cards extensively and have never had any issues. They are fast and reputable. GiftCardZen is also very good, however they don't accept the same variety of cards and they also have the weekly limits.

SaveYa is garbage as far as I'm concerned. I tried to sell ONE perfectly legitimiate GC, got declined, and can never sell to them again. I don't get it, but I would rather give my business to ABC anyway.