Hi....
Sometime that "Pay Now" button doesn't always work, try this first...delete your cache and cookies and restart your computer and try paying again, if no results then log into your PP (PayPal) account and click on the "Send Money" tab, select "Pay for ebay item/s" you might need the item number and the seller PP email address.
If you do not know how to delete your cache and cookies ,do a search on the internet on how also make sure which broswer you are using.