Should i buy a used dirt bike with no pink slip or vin number?

A guy is selling a 2008 yz250f for cheap and I want to buy it but I don't know if the bike is stolen or not. He rebulit the motor and did a buch of work on the bike for someone and he had them sign a paper saying if they didn't pay or somthing like that then he has ownership of the bike. So he has no pink slip and he says he removed the vin number because of insurance purposes. Is this safe to buy? If it is the how do I get it registered in my name? Or can I somehow get the vin number?