If you are looking to shrink your collection and please your wife, then sell. The marketplace is great though ensure you take shipping into account. While math trades can technically net you gift certificates, they are best if you are goal is to keep your collection the same size but make it "better" (i.e. more suited to your play style). Again, keep shipping in mind. I consider a $10 shipping charge to trade a game I don't like for a game I do like a great deal! Even better, though, are no-ship trades. San Diego tends to have one or two per year. If you're willing to drive up to LA for Strategicon, they should have a pretty good no-ship trade in May.
Thanks for the replies and yes I still want to keep adding games so math trades seem like the best option.