I couldn't find anywhere in their feedback forum that leaving feedback it is a requirement.

ebay feedback policy

"Sellers can leave only positive ratings for buyers. That means buyers should feel free to leave honest Feedback without fear of retaliation. We also have safeguards in place to protect sellers against unfair negative or neutral Feedback. "

I guess that if a buyer doesn't pay you could leave positive feedback, "I'm happy to say that I haven't received payment yet."

http://pages.ebay.com/services/forum/feedback.html