If I can just add a little more fuel to this discussion what peaked my interest was the mention of platforms reporting NI details as:
"Websites will have to collect data about sellers. For individuals, this will be their name, address, date of birth and national insurance number, plus what they have earned and paid in fees on the platform."
This is lifted from a Guardian article that I posted a link to on another thread. So one assumes that Ebay will require a valid NI number for any seller account - interested how that will impact those who buy under one account and sell under another.