Canadian looking to start US eCommerce business
The question of whether you can or cannot do business as a sole proprietor provided you are not a US citizen or resident is a tricky one. As far as I see, since you already were running your business as a sole proprietor you might be able to continue doing so even if you no longer live in the US, however I would strongly recommend you to reconfirm that with an immigration attorney and probably your accountant as well.
Certainly, the scenario where you form a separate business entity such as an LLC would make things much more clear. Your entity would be organized in a specific state (like Delaware, Wyoming, etc.) and you will own it as a foreign individual (but with SSN). This way you will be able to report the income you derive from this entity on our personal tax return much as you do now.
Just keep in mind - whatever you do, your accountant should be involved to make sure your income is property recorded, and all the taxes paid (both in the US and Canada).