Sorry to revive an old one, but any progress on this idea? I missed this thread somehow when I too began thinking about it a year ago or so, and I think it's the ** fundamental ** idea to the Addon community...
Pilus doesn't your valid list only work when someone gets a 2nd instance of a coin? The invalid coins would eventually get caught, but more than 90% would probably succeed when the spammer introduced it.
I suppose if you had a massive amount of valid coins, it might de-incentivize the spoofing of individual coins.
It would also increase the chances of recipients having pre-existing instances of spoofed coins in their inventory. But they are single stack items, so I don't think the numbers would really support it..
Considering this needs to be written into the core code itself - perhaps the coins could be stored as something other than items, to allow players to have hundreds or thousands of unique GUID's in their inventory.
Sorry if I'm way late to this one
Edit - an entirely separate idea is to follow the bitcoin method, and verify every transaction over the chat channels with other GHI users. This could lag quite badly though I think...