I apologise to the OP in advance for this but what are the police supposed to do? Hes seen a dodgy ad with a dodgy price and has sent a large sum of cash to a mysterious person without getting any additional information or even checking they have the car in the first place. Its tantamount to walking into a pub youve never visited before and handing £4K to a random bloke at the bar, then going to the police 2 weeks later and saying youve been robbed.
Sorry but its not the banks, the police or Pistonheads that are to blame here.
Rare is it that I disagree with you these days mate but at the point where this is proven theft the police should and must be interested in it.
No matter how gullible you've been you've still been robbed and you pay your taxes so that the police offer you an element of remedy to that end.
Totally get what you are saying about the pub analogy...... Spot on but in this case they have a bank account, eBay addresses, email addresses, fake websites that at the very least they could shut down to stop it happening to other people.
My dad would be a sucker for these things if he had access to the internet. Dumb as a plank the lad is when it comes to scams