Unless s/he violated some eBay rule, you don't. If you were in a transaction with the seller in the past 60 days you can leave appropriate (calm, factual) feedback for the seller. Be aware that everyone can see your feedback left for others and buyers are evaluated by sellers based on that rather than on what sellers leave for the buyers (which can only be positive). A seller would be less inclined to want to do business with a potential buyer who only leaves negative feedback.
