With the way these items are viewed on trading posts, any item with attributes that may give it reason to be valued less (whether they may be duly founded or not) compromises price accuracy on the same items that don't have those undesirable attributes. Same principle applies to items with more desirable traits like special levels (1, 7, 42, 69, 99 & 100). Emphasizing on unusuals which carry no special traits such as aforementioned weeds out arbitrary prices put on those with said traits.

tl;dr

Basically, a dupe can be sold for the same price as a clean unusual but it poses a high risk of being undervalued by buyers than their clean counterparts.