A 'Post Feedback' link in the 'Winner's Contact Info' and in the 'Seller's contact Info'.
This has already been done for Barry Lange (kabid.com).
Also add a feedbacks page in the control panel where each user will be able to see the feedback he/she didn't left and
those he/she left.
Maybe it would be good to add the possibility to leave multiple feedbacks (actualy the same feedback) for many users
from the Closed Auctions page - this has been done for kabid.com too (april, 2004).