I don't buy stuff online with the exception of concert tickets and a few CD's I can't find in stores. There are stores near me that have good bunny stuff so I don't really need to buy online.
For the NIC cages, you would probably do kits that can make a certain sized cage and it would include the grids, zip ties, connectors (if wanted), the coroplast would be hard to do as you can't really ship a big sheet easily and cutting it up makes the whole thing harder. I know some places that you can order each grid, so can be as many as you need for a certain price per grid.
I use fleece blankets in my cages. The ones I use are about 56X60". If you could get some about that size (or in different sizes) in cool patterns would be neat.
Bedding and litter would probably not be a good idea to have online just becasue of the costs of shipping. I buy 40 pound bags of wood pellets for about $6 and I can't imagine the cost to ship that to Canada. You could do small bags of paper based litter, but I think big bags of bedding would be impractical.
I buy hay from local farmers, the stuff I am using now was $10 for a 60ish pound bale. I know there are people who do buy hay online as they have trouble getting good hay in the amounts they need for a good price.
Oxbow and American Pet Diner are both good pellets. Since many people use them, it would be good to carry them. Both companies also have treats and other stuff like papaya and Vitamin C tablets.
My rabbits really love dried apple and banana. I have a friend who dehydrates them. This is much cheaper than the packaged stuff for rabbits and I know that there isn't any added stuff.
I think that what you would really need to do is set yourself apart from other places somehow. Many online stores have similar products. If you offered hand made toys and treats or really good shipping, more people might be willing to buy. I think what frustrates people is if they want to buy a few toys that might cost $10 and the total price with shipping is 2 or 3 times as much.