I, too, have really mixed feelings about them. Not necessarily because of how some vendors may or may not get their keys, (which is ofcourse a bad thing.)
But also because as you mentioned,
- if you use paypal, you'll automatically (?) subscribe to their monthly G2A+, which is like 5.99$, (They billed me like 30$+ before i found out!)
- If you use Bitcoin, there's roughly a 2/3 chance (my experience!) that the payment won't go through, and you'll end up making multiple support tickets to Bitbay & G2A, (which takes weeks because i'm obviously not using their ScamShield™)
Too bad that for legit keysellers, there isn't much of an alternative, and that steam is not accepting bitcoin (directly - GC, i know.) anymore... :/
I see what you mean though, a 10$ Xbox GC is 7.5 EUR = 8.6$. That's what, 14% off?
That's a lot, even for G2A. However, i could draw up multiple scenario's in which the seller could've received the Giftcard in a totally legit way, but then again, also multiple where it could be fraud.
It could be that the seller either received them in another trade (I've seen lots of users online selling Giftcards for Bitcoin @ 10-20% rates), or they might be carded/scammed from other users.
I don't think anyone can say for sure that they are/aren't.