You get in contact asking to view only to be told they're working abroad and the car is at a friend, and because that friend is doing them a favour they don't want to burden them with doing viewings. Or some similar excuse. But they've left the signed V5 with that friend so if you bank transfer them the money they'll tell you the address and get the friend to give it you with the keys when you turn up to collect.
It works on gullible people thinking they're getting a bargain too good to miss and transferring the money up front in fear of missing out. 99.9% of people wouldn't be so stupid but it only takes one for them to pocket £6k for minimal effort. FB Marketplace is chock full of these scam ads.