To add to others' advice, if anyone contacts you claiming to be from autotrader and asking if you want to upgrade your ad or they want to give you a refund for a random mistake, put the phone down and call autotrader yourself.
They tried this with me.
Regarding insurance: most comprehensive insurance companies include occasional use of other vehicles on third party cover basis.
So if they smash your car on the test drive, their insurance company will only pay third party damage/injury, not your own car.
Bear this in mind if a teenager shows up.
Always get to see their driving licence before a test drive and note their name and address.
If you can, take a copy.
And most importantly, on the test drive get in the passenger seat with keys in your hand and do not leave that seat until you have the keys back.
As others said, accept only hard cash or bank transfer.
Genuine buyer will have no objection to these conditions.