Bear in mind also that some of the fees the seller pays are avoided if the payment is made in cash. For PayPal it's around 3.4% I think (I imagine it's similar for eBay payments). On £500 that's a worthwhile saving. Which then begs the question why they wouldn't prefer cash on collection to avoid these fees...?
Also, I've been in this position a few times as a seller and I've insisted on cash on collection to protect myself. From the seller's perspective, HE is on dodgy ground if you pay via PayPal as he would have no seller's protection. So technically, you could collect the item and then put in a PayPal claim for item not received. To defend such a claim, the seller HAS to provide trackable delivery, which he wouldn't be able to, and so you'd win.
With those to things in mind, the seller should be pleased you're paying cash. In fact, he should be insisting on it! If he's not aware of the above two points, perhaps explain them and see if that changes his mind. If it doesn't, then I'd say it begs the question why he's insisting on PayPal and starts to smell like fish...