Gold bullion coins and bars, especially those issued by reputable mints such as the Pamp Suisse, the United States Mint, Casa da Moeda do Brasil, Royal Canadian Mint, and the South African Mint can be used for trading around the world. But there will be only a few places which accept these coins, and they are not suitable for low-volume transactions. In addition to this, you should have the documents / certificates proving that you are legally owning these coins / bars.