I've had this a few times, normally I send it and everything turns out fine but I can understand your trepidation with such a valuable item. I would cancel it, the worst he can do is leave a bad feedback which, if you leave a calm and polite counter comment (i.e. don't do what you say he does and start mouthing off), it's not the end of the world.
You could try a local selling group on Facebook. You will find items like that will sell really quick, with no fees and cash in hand. You're going to be talking £45 of fees on a £350 item on eBay with PayPal.