Good idea, but that doesn't mean you can't do the same thing yourself. Now if these were items that were not easy to reproduce, such as action figures, props or even handmade items (like Perler Bead Sprite artwork) great. But they're just pieces of already produced artwork or posters that just happen to not be easily available for purchase. All that the seller is doing is making/printing copies from originals or digital images and selling them, very similar to what you would find at a flea market or from those 0.99 cent stores that offer 'fake' items such as bootleg DVDs and counterfeit clothing.
I'd say spend the money to make your own re-sized posters. You'll spend a bit more but at least its something that you'll have made by yourself and you won't have to worry about it getting lost in the mail.
