You did register on eBay Canada, and that is why you receive communication from eBay.ca. But you gave a registered address in the US, which was false. You could have used your USA address as your shipping address if you wanted to, but unless you were living there, it was a false address for your registration address.
eBay does not accept the W-8 BEN form, because then they would not know what country you do have a tax liability to. Many countries (including Canada) may have requirements that eBay has to comply with, to collect or report income or taxes for their citizens and residents.
You have to register your Managed Payments account correctly, with a registration address in the country where you are resident. There isn't any way to change an MP account from one country to another. You're not the first or only person who has needed to create a new account, there have been many others. Some in Canada, who made the same mistake as you, and some sellers here and in Europe who have moved from one country to another.
There's no point in arguing about this, because we can see what country you are registered in. I'm sorry if you have misunderstood this, but this was an explanation, not a debate.
An ITIN is provided for people (resident or nonresident aliens) who need to file a USA tax return but for some reason are not eligible to get a SSN. Nothing that the OP has said has suggested that they are required to file a USA tax return, and I don't think they can just decide to file a USA tax return (probably for a small amount of income with no tax liability) in order to avoid any other tax liability and obligations that they have to their own country.
I suspect that OP has been avoiding paying GST/HST (General/Harmonized Sales Tax) on his selling fees by using a registered address in the USA (where eBay is not required to collect sales tax on the fees that sellers are charged).
https://www.irs.gov/individuals/individual-taxpayer-identification-number
