I agree on one thread at a time for each seller, number of items in the thread would be up to them. If the seller can't be bothered to put in the effort to properly track bids with multiple items in a thread(use a spreadsheet, piece of paper or Googledocs) then keep the items to a minimum. The benefit of having a fee free forum to use may be offset by a little more effort to keep track- certainly a fair trade off.
I would also suggest EVERY thread title lists the end date/time, for example Ends 11/5/14 12pm, so people browsing can determine if they are looking at a finished auction thread that's been bumped.