We recommend the following sites when buying or selling bitcoins online.

These sites have proven trustworthy in the past.
However you should always do your own research before trusting someone else with your bitcoins.
Remember, bitcoin transactions cannot be reversed.

Currently the leading Canadian bitcoin exchange by volume.
They offer convenient deposit methods such as INTERACĀ® Online, EFT and cash deposits.

Hong Kong based BitFinex is the largest USD/BTC trading exchange in the world.
They offer superior liquidity as well as 3.3:1 margin trading.
For advanced bitcoin users or experienced traders.