Wow, I miss read the OP.
I still think that if YOU buy bitcoin, be it a exchange or between two individuals, and YOU paid a fee to on top of what the bitcoin valuation, since you are a Quebec resident, you should fill out a form to declare the tax you did not pay.
For the part where you ARE the exchange, you don't have to charge tax. Just think of Canadian Tire money (CDNT) : You can exchange it for money with someone else and not be paying tax as long as the 5 CDNT is exchange for exactly 5 CAD. Now, here the tricky part : BTC/CAD change often. So you have to declare gains or loses as you would do for any financial product. Also, if you charge fee for providing the service of exchanging bitcoin you have to charge tax on those services.
You should consider writing (not calling) to Revenu Quebec to avoid suprises. Also, do as Meni and get a accountant who may know a little more about tax law that we do

Cheers