A known number has a history, and history is what car collectors love the most. Dodgy seller could say he bought the vehicle in a state of semi restoration off a guy (show him a 3 year old build thread on a forum which tells the story of where the car has been the last 20 years, original photo's proving it's authenticity etc. Buyer doesn't realise that the vehicle is still sitting in a shed cause that guy had kids and the project stalled for a while.