You are a lot morre diplomatic than I am

I recently sent this to a buyer for leaving a neutral feedback for an item that was 'smaller than expected'

Dear Unwarranted neutral feedback leaver,

I see you buy quite a bit on eBay. Might I suggest your next purchase be a ruler.

A ruler is a stick with numbers & little lines on it. It can be handy for checking the measurements that ebay sellers (like myself) put in their listings, so your item does not turn up 'smaller than expected" (not that you'll ever be buying any further items from me -as you are now on my blocked bidder list).