In general, you need to register if your self-employed income (sales) was over £1,000 in the tax year (6 April to 5 April).
Register by 5 October after the end of the tax year you started. For 2025/26 income that date was 5 October 2026. If you missed it, register as soon as you can; the return is still due by 31 January.
Since January 2024, platforms such as online marketplaces report some sellers' income to HMRC. That reporting doesn't change whether you owe tax; it just means HMRC can see the sales.
Clearing out personal belongings isn't usually trading. Buying to resell, or making things to sell, usually is.
Work out your own figure: our UK Side-Hustle Tax Set-Aside 2026/27 spreadsheet (free lite version, or the full version for £4 on Payhip) calculates the extra tax and Class 4 NI on top of your PAYE salary.
Written with the help of an AI assistant and checked against GOV.UK on 21 Oct 2026. General information, not tax advice; if in doubt, contact HMRC or a qualified adviser.