I have never traded in a game in my life.
Part of that is because I play PC exclusively these days, however prior to that I did play on the PS2 and N64 for a bit.
I just don't see any worth in trading in games unless there is a flat 0% chance of me ever replaying them, like half my PS2 games that I've though of trading in but really can't be bothered.
However, the "I had fun the first time" doesn't work for me and replaying games. Deus Ex: HR I loved the first run through, but I could never replay it. There'd be nothing to replay it for. I've already tried everything the game has to offer, and there are plenty of more interesting games out there for me to play.
The same goes for multiplayer. IMO its just replaying the same mission over and over, which is boring. I know some friends who will only buy a game if it has multiplayer, but I tend to avoid such games as the multiplayer really does nothing for me.
What gets me to replay a game is its story. If its a good story, I'll replay it. However these are few and far between these days. There are only a handful of books that I've read more than once, only a couple of movies I've watched twice, though there is a fairly large number of games that I've replayed a few times - usually because they're interactive, adding that slight extra element of randomness to the story that I can re-experience it slightly differently the next time, though the majority of my games never see a completed second unthrough.