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In Bitcoin parlance an “exchange” is an online service that lets customers buy and sell bitcoins from each other. Exchanges do not sell or buy on their own account. They’re similar to stock markets in that the price of bitcoins is set by the prices that participants are willing to buy or sell at at any given time.
The two largest exchanges in Canada are Canadian Virtual Exchange and Vault of Satoshi. As of March 28th, Canadian Virtual Exchange is doing 30 day volume of $4.2 million (~6500 bitcoins). Vault of Satoshi has a seven day volume of under $250,000 (300 bitcoins).
Note: the figures above are (of course) now out-of-date. You can check the volume of these exchanges by going to their respective websites.