You've filled out the V5 in the correct manner, given a receipt and kept a copy. Whats the problem?
You're not obliged to check out the buyers history!
When I've bought cars (especially on eBay) I've always assumed it's the seller that's the crook!!
I once sold a car on eBay for £600. The buyers address came up as the Shetland Islands. That's 650 miles and 2 ferry crossings away from me. Oh well I think that's a waste of time then. Next day I get a call from a lady to say she'll collect in 3 days time, I still wasn't convinced! 3 days later at 9pm she phones me from the bus station, could I pick her up? She's come on her own! I pick her up, drive 20 miles to where the car was, she hands me the money, we do the paperwork and she drives off into the night. Another 3 days later she leaves me feedback.
I still find it hard to believe it happened!!
There are still some decent trusting people about.