As well as the £1,000 limit Pete mentions, there is another factor. Selling your own property, which you bought to use isn't actually taxable at all, even if you sell for more than you paid. The key thing is why you bought it. If you bought something specifically to sell on, then any income is taxable, above the £1,000 level. If you happened to buy a Selmer MkVI in 1983 for £400, you play if for the next forty years and then sell it for £6,000, it's not taxable, even though you sold it for more than you paid for it. This creates a hell of a grey area.
Then there's another factor. Tax revenues dipped due to the pandemic, and HMRC have been encouraged to be aggressive in pursuing money. However, they're not so keen on chasing big earning tax avoiders, for reasons I won't go into. So they are aggressively chasing middle income tax payers, already the mainstay of the tax economy. I found this out when I was chased very aggressively for tax payments they claimed were due on Child Benefit payments. However, I had not received the Child Benefit payments, having opted out of them, as I realised in advance I would be subject to a tax clawback. Who makes Child Benefit payments? HMRC, so their own records demonstrated I had not received the money, so could not be liable for the tax clawback. But they still pursued me aggressively, with threats of fines and possible custodial sentencing. Eventually they recognised their error and backed down. For my entire working life, I've been in the PAYE system, so my tax affairs are very straightforward, but the HMRC still tried it on with me. I got no apology from them, even though for almost 40 years I have paid every penny of tax due. My expectation is they will have a target driven agenda for recovering money wherever they can, even if it really should not be due. It'll be interesting to see what sort of ruleset they decide to apply when pursuing small sellers on eBay, Etsy and so on.