Personally, I have to say I've never really done so. I've sold games to shops when I didn't like them, but I've always preferred to actually buy games new.
I buy almost all of my games used. I wait until they drop to around $12 or less for some I am iffy about and for others if I beat them, I have 7 days to exchange them for something else. This way I can play about 12 games on the same $20 bill until finding one I feel is worth keeping, like I did with Heroes of Ruin.
I'll buy new if it REALLY interests me or has a pre-order item I like, such as I am doing with Animal Crossing: New Leaf and Project X Zone.
It depends really. If I cannot get it new then yeah, pre-owned it is. Also if the price difference is significant enough (example I recently purchased ZombieU used, which was $15 off the new price).
Other then those 2 circumstances I will never buy a game used.
I prefer to get new. With used games you do not really know if it works till you try it, with new games, the chance of it working fine is much higher. I will get used if it is an older game that is not available anymore though.
I've bought a few PS1 and PS2 games used. For my current-gen consoles, all were bought new except for 2 used games, and another one was given by a friend. I usually wait for price drops, so there is hardly a difference in price of old vs. new. I would buy more used games if the price disparity were larger but with the options available to me, buying new makes more sense.
I try not to buy them even if they are at a low price. The only ones I can really remembering buying is Battlefield 3 and Pokemon Diamond. I mainly purchase my games digitally for two reasons I believe at least, its a bit more green and I really am such a green person, also because I hate having the boxes because it can be a mess having a bunch of them and I'm a very organized person. In conclusion not really unless the game is old that they don't even make anymore then I would get a used copy for a better bargain. Great topic!
I prefer to buy new (especially Nintendo for the club code) just because I enjoy having them new. If it's not a significant discount to get the game used, I'll just go ahead and grab the new since it's less likely to be scratched or damaged. But we do buy a lot of used games in general- we got our PS3 just a year ago, and the used games were a boon! And with my never having played a lot of the old DS games, I've bought several of those used as they're super cheap to get.
It depends on what game it is and how much it initially costs, but if it has a pre-order bonus that interests me, then I get it new. This also applies if I really enjoy the franchise. I still like to get at leadt one spare just in case something happens to the main copy; this is where I buy used games.
Mainly my retro games I buy used, last game I bought used was a collector's edition of Ocarina of Time for the N64. I bought a few used for my GC, and Wii.
I like my games fresh from the manufacturing company. I don't want some dinky half-chewed-up game that has food gunk stuck to it.... The newly-bought games don't have that marked-territory scent to it; it truly feels like it's yours.