Delays but does not prohibit.
Lets use the Kiba as an example. If I wanted it right now, I would have to challenge Shadow.
If he's active, great, that just means I have to send him a PM and set up a fight. But what if he has made his character so overpowered that he's impossible to beat without resorting to his level? Oh well, just give up and let him have it or wait for him to slip up and fault him on a clause.
If he's inactive? Well, I have to wait the 60/90 days in order to fault him and claim it by default. That's a lot of waiting. Plus, if you don't call dibs first, you wasted all that time waiting.
If copies are allowed then all you got to do is contact a gm and, considering how often they're on, expect a one/two day response. They can start an rp in the zones or somewhere and get the ball rolling.
Sure, you may lose out on the fun of tracking down another player and challenge them for a unique item, but how often do you really see people use those items as anything more than a status symbol? Loads of people opted for the custom items and crazy jutsu route because a lot of these items are underwhelming in comparison. Personally, I'd love to see someone that builds their character around a certain items or weapon, but that seems to be something that died out sometime around the Twilight days of this place >_> That's why I'm trying to think of solutions on how to fix these things.
Also, I just used you as an example Shadow because Kiba was the shortest to spell and I'm lazy