What's preventing someone from purchasing a GPL/BSD license for the source code, and then hosting it publicly (with licenses intact) somewhere, say, github?
Nothing and the author is pretty clear about that. However the games seem to be free so getting the source would mean the author gets paid something. It also isn't clear if the game assets (art, sound etc) are covered by GPL/BSD or a different license.
All open source software is half off this weekend. And every weekend. Odd numbered Tuesdays its always double price. A friend once got open source software for 99% off, although I've had to pay triple. But not a penny more.
Most (all?) open source licenses place no limitation using the license and selling the source code.
Nothing. As it says on the website you could even slap your own logo on it and resell the software under your own brand.