I'd like to contribute a few datapoints that some may find useful.

ABC: Tried selling legitimate cards I purchased during a staples deal to ABC, they accepted the order online, and I even called to verify the instructions for shipping the cards to them, but they held on to the cards for 3 weeks before I got a nasty email telling me that though the balances were intact, they were "high-risk" and they would be sending them back to me.... They waited two weeks to ship. I will never work with them again

Junkcard: I'm a bulk-seller with them. Horrible customer service, extremely slow to verify cards and payout, but at least their slow service is predictable.

Giftcards.com: Doesn't matter because they are SaveYa now

Saveya.com: I'm a bulk seller that was rolled over from Giftcards.com. Very meh. Not particularly pleased with their customer service and their VERY consistently changing rates (usually seems for the negative.) Also, they changed the rate for one of my cards AFTER I was shown a different offer price (thank goodness I screen-shotted it) and they blamed it on a tech glitch. It didn't exactly inspire confidence. I do know my experience BUYing from them has been technologically glitchy and led to me being "double charged."

Giftcardrescue: Pretty much the worst and dirtiest of 'em all. Everything about their transactions, guarantees, and policies screams don't do business with me, which is probably why they have to offer such high rates to entice people.