Ops my bad.... I checked the code but didnt find it. Maybe my subconscious mind read it and gave me the idea...
Nevertheles, a pawn shop and a Jewelry/Gem one is a good idea from the story point of view.
Here is the code if someone is interested (or want to document it in the wiki, you get 2/3 of price if the shopkeeper don't sell items in the category)
{ If this part matches the category of the shopkeeper, it's worth more money. }
{ Actually, it works so that selling inappropriate items are penalized. }
if not ( ( Part^.Scale < 1 ) and PartAtLeastOneMatch ( Categories , SAttValue( Part^.Sa , 'CATEGORY' ) ) ) then begin
Cost := ( Cost * 2 ) div 3;
end;