It's a 3-way win-win-win situation, pretty much.
If you're able to buy games online, from a site like Amazon or eBay, you're able to get games and accessories at lower prices compared to normal retail stores, through TBT's site. Also, the profits TBT makes from sales through our site will most likely go towards stuff that will probably reward members in the future.
As for TBT, in general, every sale makes money for the site. These extra profits will be used to better the site... we've had a couple ideas how to use profits we make, and I'm sure we'll have more in the future.
And, of course, Amazon is advertising their items somewhere else, which is obviously good for them.