This G2A exploit was leaked from a private group.
Tried to report this issue to G2A with no success.
So basically it exploits a vulnerability in BitBay payment system to manually expire tokens and get instant refunds when paying with bitcoin.
I just tested and got 4 Amazon gift cards for free.
The steps are in the provided PDF below.
(Remove the empty spaces before and after the dots to access the webpage, links were not allowed).
https://easyupload . io/4atzxc
http://bit . ly/2XGszWC (mirror link)