Dear Mark,

I wouldn’t say an LLC or a US bank account is absolutely mandatory in your case. You might however need to obtain EIN if you sell using PayPal.

If you run your business online from Canada and sell to US customers using Amazon FBA services what you might need is to register a Sales Tax ID in the states where your merchandize is physically stored (please read this answer to better understand how the sales tax works in FBA case).

Of course if you do a lot of business in the US at some point it might become more convenient to do it under a US LLC, in which case you would need a bank account and an EIN (federal tax ID) for that LLC.