It’s just a simple marketplace, like eBay - you commission a service and the website takes a cut of your payment. Of course it’s legit, since the transaction takes place between 2 individuals, as a company they’re not at all liable for fraudulence.
Though your message/invitation is undoubtedly a ‘cold call’ likely by the site founder himself. You’d probably be better off advertising in your signature / profile.
The crazy part of all this is the guy thinking he can make $20 for a single Forza livery - lol. 0 Reviews, naturally.