'Ebay has been half my income for years and declared, so no i,m not worried about hmrc.'

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Well, HMRC don't actually care what sort of account you sell on, as long as you declare and pay any tax due!

So yeah, *they'll* be fine with you 👍

But HMRC aren't the arbiters of who is or isn't an online business in the UK. That's down to UK Trading Standards. 

And they say you're a business.

OK, T.S. are a bit quieter and don't arouse the terror that HMRC do!😱  but if you were unfortunate enough to get reported by a clued-up or vindictive buyer for something , you'd find yourself in the soup.

But even if you keep totally below Trading Standard's radar,  I'd be getting worried about the future of A.I. and computing etc..... when various govt depts start to link up* their information using A.I.,  it will become obvious who is up to what .

(* some people seem to think this linking will not happen: I'd bet loads on it happening within the next 5-10 years...)