This may sound sad, but I would never get rid of a real favourite (even if I no longer played it). I've got many a game going all the way back to the spectrum, and Atari ages. And every now and again, I get out the old systems and have a blast! (Then they go back in the cupboard for a few more months).
Any game that didn't really capture my interest, would usually end up on ebay, or traded in within a few months. After that the 2nd hand value drops so low, that I'd prefer to keep it.
Am I really that sad - or are there others like me out there?