Of course you aren't,.

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"I’m a private seller and not a business LOL. Let me know if companies house says otherwise or any official registrations."

This seems to be a misconception, because you don't have to be registered with Companies House to be a business.  Businesses can be sole traders, partnerships, or non-limited companies, as well as limited companies.  Even the smallest one-man-band is a business if they are trading.  Just sayin'

"EBAY WILL BE OUT OF BUSINESS SOON!"

I'll take that bet. How soon is 'soon' and how much do you want to stake?

Their response made perfect sense in the context of the post and person they actually responded to.  Which was not your post, or you.

I've just tried Vinted (with clothing) and I thought, back to Simple Delivery on eBay, all is forgiven.
And as a buyer I get so frustrated with the lack of details, wrong sizing and Vinted's stupid measurements system - across shoulders and length... across shoulders is useful but not until you know the chest measurement.
I couldn't print out my label and arrange collection via Royal Mal - which eBay offers free - and had to go to the post office with a qr code...