Users can buy or sell bitcoin through the interface. How Can I Buy Catcoin? However, More Material you may be able to buy Casascius Coins from others on eBay for such currencies. Of course, since the face value of the coins depends on the integrity of the embedded key code, you should only accept Casascius Bitcoins bearing an undamaged Casascius hologram from others. Each piece has its own Bitcoin address and a redeemable "private key" on the inside, underneath the hologram. There is a mathematical relationship between the Bitcoin address and the private key inside the coin. The digital bitcoin is actually located on the public "block chain" stored on the internet, but it is completely inaccessible to anyone unless the private key from the coin is loaded into a Bitcoin wallet. How they work: The "private key" is on a card embedded inside the coin and is protected by a tamper-evident hologram.