updated.........
A system like that would be fair if the person was selling comic books, stands, loose clothing, etc... But if someone was selling their whole collection of 1/6 Star Wars figures...then I think they deserve a +1 for each figure. You can even run into gray area with clothing...if it's a custom outfit that someone hand made does that deserve a plus one per item...vs. someone that just has a huge lot of clothing they picked up on the cheap? I feel custom stuff deserves a +1 per item, whether it be those cool stands with posters, a set of heads or clothing.
I appreciate people's input on the matter. This will definitely help us figure out a better system.