I've had a had a guy use a printed piece of paper folded around a game, labeled it, and mailed it to me on a trading site once. That's got to be the worst one.
Not only that, but the game had what looked to be a post-it note on it that was glued on and not coming off without significant ugly-tearing remains; which he never mentioned. Bad trade all around. When I confronted him about it, he simply replied "I consider it in "very good condition" as long as it works".
But that's the risk you take buying or trading online. Unfortunately to get the games most of the collectors like us want, it's almost essential to buy online at times.
Personally, I ALWAYS use either a bubble-mailer or bubble-wrap. Even if it's too awkwardly shaped for those methods, I find cushioning of some sort, still, even it's just a bunch of wadded-up plastic bags.