"BY OFFERING ALTERNATIVES WHERE A TRACE CANNOT BE MADE" Of course it's a scam; or rather, facilitation of the scam perpetrated when a crooked vendor sells a car with a bogus full history, which is when the con takes place. The other alternative is to buy a blank service book off eBay, and a made-to-order garage stamp (also off eBay), and falsify your own. Bit like the difference between shop-bought and roll-your-own cigarettes...
I complained a while back to eBay about these sort of adverts, which plainly have no purpose other than to facilitate dishonesty, but got nowhere.