I have the same problem.  There are some vendors I already own over 50% of their items.  Actually what might increase sales is to work it so loyal customers of certain participating vendors can get a discount for that vendor only on remaining items.  It would appear to me that would be a winning situation for the vendor, since he can target those customers that are already fans, and have some guarenteed sales for any new items that come out.  Anyway, it is something you might want to think about.