I send an email like this:-
Hi
re: Ebay Item No 2187745455 Green Widget with Bells and Whistles
Just to let you know that I have left you positive feedback - the Widget arrived this morning and is just what I always wanted!
If you would like to leave feedback for me, please click here http://cgi2.ebay.co.uk/aw-cgi/eBayISAPI.dll?GetAndShowTransactions&item=0&nflw=1&page=1
Seems to work
[I got the address from the Ebay Toolbar - the jury is still out on whether it is useful or not!]