I don't really sell or swap items very often, but I do give things to my friends. I wish I hadn't given my 180 brush to my brother's girlfriend, I wish I kept a few paint pots.
I don't give away palettes or eyeshadows much. I've only given away doubles of singles I've had, and a few lustres that I couldn't get to work. =P I would never sell or trade or give away a Mineralize Skinfinish. Even if it didn't really work well, I know I'd regret it!