Belkeve it or not they have fixed one aspect of the search. You can now find items by seller's ID. If you click on the box indicating By Seller and then scroll don the page to by seller's ID.
This shows the quality of Ebay programming as you can also do this from the "Find Items" screen.
You still cannot do anything with the "By Bidder" search - this has been inoperative for approximately 5 years. A clever programmer would have only a find items box and in brackets also by seller and Item number) as all three searches are accessable from the find items screen.