I've still got my copy of System Shock 2 in its cellophane wrapping, so it's in mint condition. I'll never sell that, along with my big box copies of Thief and Thief 2. Also keeping my copies of Crusader: No Remorse and No Regret.
I have sold a lot of other big box games recently on ebay though. They don't go for much though, unless they're true collectors items. I'm not sure what to do about some of them which I don't think will sell. I'm loathed just to throw them out.