I feel you have a much more valid point PKMNB0Y. Also, note that Jangel, you did note it having nice stats or something along those lines. The fact that increased damage to "dark creatures", which we haven't even fully decided if we are having such a classification of monster yet, is something that would definitely add rarity. 

Enchantments/Enhancements (I don't even know which it is supposed to be anymore), are classified to a set of things. Slicing, Durability, Heaviness, Quickness, etc. There is no enchantment/enhancement that improves monster damage. Weapons can be made with such a bonus, I BELIEVE, but can't clarify this one myself. They can't have a normal stat weapon that you enchant to make it improves damage to a specific monster. 

Also, 150-200 col? Well, either you got a super bargain, or it's absolute crap that shouldn't even be sold, cause realistically, that's hardly anything in the game. Probably worth three potions or something like that.
Hell, a peace offering of potions would be worth more, if that's the case of it being worth 150-200.

Well, none of this matters, as that katana is going to break.