I'm not quite sure what I changed, but I am able to buy 'green' and 'orange' (set and unique? respectively) items from vendors.
I think I set the following to:
ChanceForSlotToImprove = 10000,
ChanceIncForSlotImprovePerRareLevel = 10000,
[Edited by SignupSucks, 10/31/2008 8:36:48 AM]
By setting the traderlevel...or something like that...to 10000 you get orange and green items.
The green and orange items start appearing around level 18.