Here's where the MD5 system fails:
Create and item (say, "Potion of Healing" that gives +10 health).
In game, buy a Potion of Healing, save game, exit.
Edit item in editor (say, change potion to +20 health), save.
Load saved game. Now, if the game saves the items' effects, you have two Potions of Healing in the game. One gives +10 health, the other gives +20 health. Apparently, you don't see a problem with this. In every other game I've played that allows users to modify items, the changes apply to all copies of an edited item in the game. They do not generate a new item that is wholly separate and distinct from the original. The system you proposed breaks players reasonable expectations.
I could see using an MD5 method to generate a unique ID the first time an item is created in the editor. Subsequent changes to the item should not generate a new ID, unless the item is specifically saved as a new item.
I vastly prefer user generated ID's in a well defined name space, though.