He may have invented the french stock to exactly avoid what you're trying to do. I really hope he's making some money otherwise what is the point and so what. You find a guy who's selling Trophy new pair at €600 that you'll never find anywhere else and you're asking for invoice, place he's buying them from and so on. Get real guys!
Any 182 cup can use those, the market can be huge and Holland is a large Renault market. Seems to me that you wait any longer and this stock which has reduced from 20 to 14 sets may be gone in a matter of days. Just consider the guy as an individual, ask him for detailed pictures and go for it.
He could have advertised them at €800 and you would still be wishing to buy so go for €600, forget group buy. The guy is on facebook, he's a pilot for a small holland racing team, he's associated with a Renault Sport specialist (does he work there? That's what he says in his FB profile), you use some insured payment (paypal, bank transfer) that leave traces, you can contact the guy, I'm not saying it's 100% guaranteed but at least you have enough data if things go wrong.