As far as i know, they shouldn't. G2A sells keys they bought from distributors like myself.If you check the fallout 4 page you see you only have one offer, which is G2A itself.
Kinguin are marketplace sellers, so i cant exactly say.
But what i have heard from other code traders is that Kinguin fines marketplace sellers pretty hefty if they dont supply pre-order keys. Something like 500 + a certain procent of the sales amount (dont know for sure). However not everybody can sell pre-orders on Kinguin, you need a good feedback rating.
Professionally.
Just out of curiosity, is there any reason why Kinguin and G2A don't approach the publisher themselves rather than a distributor like yourself? For a big title like MGSV or Batman it seems it would be easier to buy straight from the publisher rather than collect them from a wide range of distributors.