Most fraud about car sales is to claim a lower price in order to skip on sales taxes collected by the states' motor vehicle agencies. Not all states charge a sales tax on individual-to-individual sales. Here in Kentucky, the state constitution says that taxes have to be charged on the assessed value, so part of the annual registration is based on the assessed value (min $100 for boats or $200 for cars/trucks). I used to work for KY's Transportation Cabinet (combo DMV + highway dept).

I don't understand how this changes anything?

How would they know now?

These examples are silly, most people are not selling a car privately all the time and they can handle any reporting or changes when you transfer the ownership.