In addition, you have 60 days to leave feedback. The most effective feedback is calm and factual.
"Not shipped after 5 weeks. No refund rec'd" (You are not allowed to mention PP or eBay disputes in feedback, btw.)
When you leave feedback you will also be asked to leave Detailed Seller Ratings. The normal Rating is 5 stars, the lowest rating is ONE star. No stars is a free pass.
When a seller drops below 4.3 Stars, her seller account is restricted and is becomes more difficult to list or sell on eBay. This can last for up to a year.