Most people buy locally so your shop has to be in a location with sufficient demand for raw materials. Alternatively offer low cost shipping, but that is often out of your control via couriers and domestic mail, and materials are heavy so usually they cant be very cheap.
Metalsupermarkets offer free cutting services also, unlike some other suppliers that may charge a price per cut. Its just not a waterjet cutter but a standard metal bandsaw from some of the ones ive been to. However they only do orthogonal cuts to the workpiece. If you offer to do rough cuts for more complex geometries since you have a CNC waterjet, then you will have an advantage there. For example a hobbist may want a triangle shaped rough cut, so if they buy from metalsupermarkets, they buy a rectangular piece and cut to two triangles himself. If you offer rough cutting servces for more complex geometries, you may be able to cut it into the rough triangles he needs.
But, if the customer has a saw himself anyway he may not need this serivce, in which case it goes back to cost and convenience again. Whos cheapest and closest.
One thing metalsupermarkets dont stock are more exotic alloys like titanium alloys and nickel superalloys etc. They also dont stock plastics. However you can order both from them.
Which part of Canada are you setting up a shop?