What are you not understanding??

He has not changed the car into his name, he is not the registered keeper......

Your buying a car in a private seller off someone who is not the registered keeper! the registered keeper will have a red log book!

He may / may not own the car - why not put a deposit down so you only lose a small amount and secure the car whilst he becomes the registered keeper or runs away with your money...

The car not being reported stolen is of no consequence, it may be his mates who has the red book and they may be splitting your money 50/50 and calling the police to get the car back by reporting it stolen!

The Blue Log Book is NOT worth the paper its made from as thousands of unprinted log books got stolen!