They are a grey key seller, i.e. they re-sell keys/gifts, etc. They are not (AFAIK) an official reseller in anything.
They have a open market place (with a rep system) where you can sell your own keys. This can include region restricted games too, but the region restrictions are usually labelled.
And.... they charge a fee on top of the price (around $1)
I have bought 40+ keys from them (can provide proof if need be), no problems.
They do have a guarantee system called G2A Shield (at optional cost of around $1) where they verify the key has worked, using a system similar to Teamviewer to watch you activate the key. I doubt they will refund if you do not use this!!
The only issue I have had was with a refund. I added 2 Titanfall Season Passes to my basket by mistake, anything over $8 I use the guarantee system. And I explained to the live chat rep that I had paid for 2 copies in error.
They said I could get a refund on the spare no problem, and should only take a couple of days. It took just over a month to actually get it back. But I DID get it back!
So yeah, they are safe to buy from, BUT make sure you triple check everything before hitting that pay button.
When buying from G2A, check the following things:
-Region restrictions (read the item description)
-If a Marketplace seller, check the rating.
-Remember to add the cost of Shield (optional) and the transaction cost (not optional) to any listed price before going through the buying process, it will save you time.
-Check the cost all the way through to final payment.
Hope that helps.