There are several exchanges on the Internet that host trading of bitcoin 24 hours a day, 7 days a week. With all of the buying and selling on the order book, just like in the stock market, a price is discovered. Right now, in 2017, the prices on the exchanges do not match up together as much as they used to. In fact coinbase, one of the easiest exchanges to buy from in the US, has one of the highest prices.
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