Here's the link to the HMRC guide on this:
https://www.gov.uk/government/publications/selling-online-and-paying-taxes/selling-online-and-paying-taxes-information-sheet?&utm_source=t.co_hmrcgovuk&utm_medium=social&utm_campaign=online_sellers
It's not really all that helpful I feel. Lots of "If this is you then you may / may not be likely to be liable for tax"; no real clarity.
But I got involved with a broad thread on Twitter (yeah, I know, X) about this and I'd be interested if people feel this is a fair summary of what the guidance seems to say:
1. If you are selling your personal possessions, and doing it as a one off thing, you're 'probably' not liable for tax.
2. If you buy things for the purpose of selling them, then you are probably liable for tax.
3. If the income (not profit; income) that you turn over by selling online exceeds £1,000, you 'may' be liable for tax.
4. If your online selling income is below £1,000, then you 'may' not be liable for tax.
To be honest, I think it's all going to become part of the huge data set the government seem bent on developing. Palantir are taking on all of our NHS data (whole different topic of course). The government are legislating to have access to all the bank details for anyone receiving benefits of any kind, including pensions. All the bank details; not just how much you get via a specific benefit. And all of this online selling platform financial data could well be yet another area they feel they can use huge tech companies and/or AI to dig into and find unexpected uses for in the future, tax being just one of them.