I've never bought or sold used brass. I'm just a little leary about how many times the "once fired" have actually been through the cycle. Not like there's an odometer on them and I ain't gonna' check every single case for dings, cracks and big primer holes.
If you're buying, I'd recommned just ponying up a little more bucks and get new brass or loaded ammo and re-load that brass.
If you're selling, shoot the crap out of it and sell the stuff after you have problems with head / case seperation.
Just kidding on the selling deal....honesty is the best policy.