G2a.com screwed me a couple of months ago.
I bought a Steam key for Rust, at a fiver it was cheap but it didn't work. It then took me three months of having to chase them and getting the run around before they finally agreed to give the fiver back. For most of that time they were still selling the game for a fiver but by the time I got told they would sort it out and that I was getting a refund and not another key (obviously I still wanted the game) the game had gone back to being over double what it was.
It felt like they were trying there luck and thinking "it's only a fiver, he'll give up if we ignore him for long enough", I kept phoning them and kept being told that they had to send it to so and so and I'd be contacted and then never was, I'd wait up to a week or two for them to get back to me and everytime they didn't, It happened 4 or 5 times and took nearly three months.
Use G2a at your own peril.
I've done lots of purchases through Kinguin and GreenManGaming and had no problems, Actually I had one with a code for the Far cry 4 game but I was given a replacement code within 2 days so I don't mind that. It's the waiting for months and having to chase them that is wrong.
Considering that I'd paid G2a there sales guarantee fee that they offer with each game purchase I think that this situation was unacceptable and no longer use there site.
Be warned, It may not happen that often but when it does there sod's.