Okay, I posted the HMRC guidance earlier. The AI summarises thus:

"HMRC defines an online business or seller as any individual or entity using digital platforms—websites or apps—to sell goods, provide services (like taxi driving or food delivery), or rent out property. This includes "side hustles" on platforms like eBay, Etsy, or Vinted, requiring reporting if income exceeds £1,000 or if 30+ items are sold annually."

It is therefore my humble opinion that if you have sold more than 30 items in the past year or £1740, then you really should have a business account on eBay. In all cases, over 30 items, your sales figures go into HMRC from eBay automatically and you may (or may not) be taxed accordingly. I believe HMRC will look at a number of metrics such as if you sell the same kind of thing over and over again and with your historic feedback rating and date of opening your account, I am sure they can come up with an assessment. If you have not been filing tax returns for your activity then you may (or may not) get an assessment for years gone by which it is then up to you to prove wrong.

There have been a few people try to claim they saved collections eg Vinyls up over many years and now this is them liquidating, or they inherited property (presumably declared in the IHT statement) but these have not washed. So it is better and safer to get your house in order right now and change to a business seller account and ensure your tax returns are filed appropriately.