If you bought DC Universe Online on PS3 and decided not to commit to it for the usual MMO lifespan of forever, you won't be able to recoup your losses by trading it in. At least, you shouldn't trade it in, even if you find a store who will accept it, or you risk being a super-jerk. Sony Online Entertainment confirmed to Lazygamer that the serial key included with retail copies of the game is a one-time-use thing, and each disc only includes access for one account. That means that the first person to use a copy of DCUO is the only person who can use that copy.
This kind of restriction isn't exactly new to consoles, with trade-ins of every iteration of Phantasy Star Online refused by retailers due to the unique codes tied to each disc. But it's not common -- mostly because MMO-type games on consoles are so uncommon.