The current site templates do not allow any direct commerce, in the sense of having a shopping basket or taking orders. That will be available in an upcoming version of MyBandSite.ca.
But if you are already selling your CDs or other merch on external sites (like iTunes or CD Baby, for example) you can easily link to those sites and keep on selling your stuff. MyBandSite.ca does not take any cuts from what you sell that way.