if he pays by some dodgy method, then claims the money back.
like moneybookers or an escrow service, etc.
the only things you can rely on are bank transfer and cash. if someone bank transfers you money then says they didn't then the banks insurance covers it neither party loses out whilst they investigate and usually if they have lied the bank won't re-imburse them. however the bank does warn them in these cases it's likely they will know who transferred the money if it wasn't them as the security is so good.
you need to know passwords, pin codes, etc.
so he could be scammed if a dodgy payment method is used.
however he is advocating going outside of ebay. so he is the one trying to circumvent ebays own protection. so the seller is covered if they try and play that card later down the line.