It goes against the carfax terms of use. If you post a vin, that unique string goes all around the world and carfax will notice the coinky-dink and could bust the subscriber. If the subscriber is dumb enough to "be nice" they have it coming. NHTSA has links to one-time-use title search companies for about $3, BTW.
I wouldn't trust a dealer's carfax either. They can photoshop. Carfax is one very small tool to be used in checking out a vehicle.
In my state a prospective buyer can just demand to see the title! It's so simple, yet noone does it. Watch the dealer squirm as their lender might have it etc but the lender needs to understand state law! It won't catch "everything" but it will also show if a car was repo'd, which carfax doesn't (always)!