Did you "merge" the sellers cards into your "legit" card? NEVER do that. They will close any card that has any (cough)cheap(cough) cards attached to that they find suspect and you also lose you "real" funds. They can easily follow the transfer of cards and per their terms of service regarding gift cards you are SOL. That's the chance you take by buying this stuff. Win some, lose some.
I merged one $50 card from here onto my real account which had $10 or so. used it for like 10 days. Did not experience any issues at all. Maybe I just got lucky.