There is no Paypal account number. The account is called by the email address you open it with.
There is no danger in giving the seller your email address (outside of phishing questions) but as noted, the seller should already have this information.
Tell him to click on Send Buyer an Invoice. If he is new, he may not realize that eBay has already sent that (if , of course, he entered shipping fee information when he set up the listing). However, the invoice form will give him all the information he needs.
