Easily the best and most pleasant game trading/selling experience I've ever had was with Reddit's Gameswap
subreddit. There's a system for showing who has done a verified and successful trade, you can go back and see all of their trades and which are successful, and overall it's really easy to see who's reputable and who's not. Usually newcomers like yourself are expected to ship first in any given trade, so you should pick someone who has a good history for your first few trades.
Another option I've liked in the past is Goozex
, which lets you trade in a game for points that can then be traded toward other games. The fact that it turns your traded-in games into a middleman trading currency is nice because you don't need to trade one game for another or even find someone who wants to pay cash for it, so it's pretty convenient in that respect.