In the simlplest terms I viewed this working like our soul stones do now.
Before activated... anyone can hand it to anyone. But once activated it simply wont go into the trade window anymore.
Would adding that to every equipable item in the pallet really be that big of an undertaking? *hides in his nuclear fallout bunker*
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Problem with this is...suppose you have a selection of weapons, armor, or even clothing that you wear but for reasons of inventory space or weight (or any other reason), you may not want to carry them around all the time. By making it un-tradeable, you also make it undroppable, and that means you can't put the item into any sort of storage (bank chest, persistent housing storage, etc.)
This would eliminate the "cottage industry" of people who design and customize clothing and armor for other people, because only something attuned could be worn and only things that are worn can be customized.
You also make it more or less impossible to get rid of old items you don't want anymore, unless we make even more items and systems to destroy "attuned" items, potentially cope with administrative problems of "oops, I messed up and destroyed the wrong item..." occurrences...and possibly other things I haven't yet thought of.
NWN doesn't provide a simple way to say "Only Character X can use this" on any given item that's reliable and that isn't also a burden and a source of difficulty to the person who should be able to use it.
Can it be worked around? Probably, but is it worth the time, effort and pitfalls? I'm not so sure, especially since that's not the only or even the biggest thing wrong with the game economy.
I am sympathetic to the laments of crafters who are undercut by people passing items on rather than people needing to buy everything from a crafter. I eliminated almost all craftable end products from the drop tables a while back to help crafters. Unfortunately, something like item breakage and attunement/soul-binding is something that really needs to be designed into the overall system rather than just slapped onto an existing one.