Steam

It's not because none of the current platform let you do it that it's a bad point. People have traded games for decades before digital platforms, it wouldn't even be innovation lol. You can suck on Steam all you want, it's just your usual capitalist business, they dont care about you and will fuck you up the very second they evaluate they can make more money by doing so. But in the current state of things, they basically make tons of money by doing almost nothing (providing server space, wow) and "gamers" will rip their shirts off at the slightest criticism of that company.