Why, in swag heaven of course. Swag heaven being eBay.
Since most of you have never been to E3, let me tell you what happens once the showroom floors open: there are hundreds of people running around the booths collecting as much shit as they can get their hands on. T-shirts, lanyards, posters, fridge magnets, pens, whatever it is, it doesn't matter how big, how small or how awful it is, if it's got a game logo on it, these people cram it in their bags.
And when the show is done? That crap has to go somewhere, and eBay is the place. While it's a little sad this practice exists, since it's all essentially just marketing, there are still plenty of people at home who would love to get their hands on a poster they couldn't otherwise get as a man on the street.
Note that we of course don't ever do this, because we never collect the crap in the first place.
E3 2011 [eBay]