If you're trading them back in, I'd just keep them in a sealed bag. If you're buying them to play games with them, you could take them apart in order to clean them. Of course, GameStop won't take them in for trade or return at that point.
Right. One other thing about buying used controllers, ALWAYS BUY THEM IN STORES!!!. Trust me, especially if you know the person working there. They'll gave you a like-new controller for the used price + theyll let you test it out. If you buy online it's a hit or miss.