This bug is fairly simple , It works similar to the backspace scam but it's much easier. Say you want to trade 2 items, you put your item in the trade box and wait for player 2 too put his item in the trade box. Before you can accept you accept and then drag the item back to your inventory right after accepting. The accept will not cancel and you will get his item and keep your item! The bug is that after you modify the trade the accepted does not go away And you are still ready to trade! Does indeed work on the new 3.9 patch Confirmed and I don't think you should be able to get banned or so , atleast it's pretty hard to spot it!
Edit: It can also be done much easier by waiting for Victim to accept first and then you move your item out and accept.