Since Bell has introduced some GSM phones, why is that if you get an unlocked GSM phone from eBay, it will not work for Bell?
http://support.bell.ca/en-on/Mobility/Network_coverage/What_is_the_Bell_network_coverage_in_Canada
This handy tool will tell you the CDMA, and HSPA+ network coverages.
The new network for Bell is called the HSPA+ .. its a bit harder to get an unlocked device to work with this network because it is newer, and the devices need to work on a specific frequency i believe to get the service working on your "grey market" device.. (such as voice calling/data connectivity etc)..
if you want to know the compatibility i would highly recommend getting the IMEI serial number of the device you want to purchase, and then calling into Bell Mobility Client Care and getting them to check the compatibility.. :)