Right now you can get Crysis for $7.49, Left for Dead 2 for $9.99, Battlefield Bad Company 2 for $14.99, BioShock for $4.99, Half Life 2 for $4.99, Star Wars Knights of the Old Republic for $4.99, etc etc.
Steam is definitely good. The game trading aspect is irrelevant here in the UK because a lot of places do not take used pc games, even if you brought it back unused but you have taken the wrapping off they won't take it.
Even then a lot of the places that do take in the 2nd hand games ask for it to be in it's original plastic packaging, unused, and they still give you a used price for it.
Trading between friends/family is really the only way to get rid and get something in return.