Some games I have returned and regretted then later bought back for various reasons like the GC version of Twilight Princess. I traded in the GC version for the Wii version then later regretted it and wanted the GC version back.
A few pokemon games, Skyward Sword, Minish Cap, Sunshine, Other M. Although in Other M's case, I got it back pretty cheap and realized that until a better 2D style metroid in 3D comes along I might as well enjoy Other M.
I traded in Brawl not too long after I got it. That tripping mechanic and the slow gameplay left me wanting.
Lube Me Up