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What they are not telling you is that this same certificate can be issued to several other customers. So they effectively sell the same bar to many people. This scam can only work so long as not everyone decides to cash in their certificate and demand their physical gold. And in fact the vast majority of paper gold investors never bother to cash trade in their certificates for actual gold or silver.
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So we should use blockchain to control gold and silver market maybe not whole market but certificate part.
Using the blockchain will allow to control individual bars deposited in any treasury and prevent you from "selling" the certificates of the same bars to several people at the same time...