vicklemos: For those who keep games for trade purposes and all that's a direct hit. But there are also some maniacs (;P) and all who keep games in their inventory just to, well, keep them.
It's like a new way of storing games sealed - daf*q?!- and all in the craziest sense possible.
That's not really it. There are people on steam who are also generous like the way GOG is, running contests and giveaways, where the gifts would obviously need to be stored in the inventory first. And ironically, actual gifting is impacted, as most people (those who are savvy anyway), tend to buy gifts on sales, where they keep and gift the game to their friends/folks..etc.. on the right occasions like Birthdays, Christmas, New years.. etc..
@Topic
I'm not sure how this would affect G2A. They deal in everything grey market, not just a certain part of it. And while I'm not an expert, keys seem to be a bigger portion of the equation. If I'm a buyer, I'd prefer keys. Easier to prove if scammed. A key is given to me upfront or is not. The key either works or it doesn't. Rather than trading shenanigans.