After much faffing around I finally get it! G2A is like AuoTrader not Amazon.
If you have any issues with a third party seller you are on your own. G2A cannot help.
Buying from G2A is fairly straightforward although their is some uncertainty around their sourcing of game keys.
Their is a subscription button and fee's for inactivity, also a transaction fee which increases the total price (on new titles, you'll save pennies, not pounds) so check each page before you proceed.
If your buying from a third party, check the reviews or your going to get burned.
Bottom line, buying from G2A direct is like buying any second hand product, some risk. If your buying from a third party the risk increases so you need to thoroughly check out the buyers reviews.
Compare prices as they are certainly not always cheaper. Summary, G2A is a honey trap, behind lots of fanfare and show its on par with online Poker sites a lot of effort has gone into dressing up a second hand site and hiding lots of hidden charges.