Sometimes I send a second invoice. Sometimes you get people who take for ever to pay. If you don't get a response "Open an unpaid item case" with Customer Service. The thing that works for me is try to stay calm and cool and don't become agitated or threatening in your correspondence. Be polite and professional.
Work with Ebay Customer Service and the buyer. You might have to cancel the purchase. You can then make a Second Chance Offer. Fully understand what your doing before you act.
Just like our forum, Ebay has the information available if you spend some time searching the Customer Service section of the site.
Good Luck,
Chris
AKA: BigBlue