RPG books you returned...

[Achan hiArusa]

Also, not quite a return but I bought a bunch of White Wolf modules for less than a dollar a piece and took them to the used book store and sold them for 25% cover. Made enough money to eat that week (don't miss that level of poverty).

 

[SteelDraco]

I always had problems with the bindings of GURPS books - they tended to crack at the spine and start losing pages easily. I think that was the only thing I've returned. I worked at the FLGS at the time, so it was trivial. As I remember, it went back defective.