This has been the tax rule for years.
You have to be TRADING with over £1,000 of income to have to do the tax return. Selling your old clothes is unlikely to be considered trading (unless you are enhancing them first, keeping them for a short period of time, or have a high volume of transactions).
Even if you do meet the threshold for trading, and generate more than £1,000 of revenue, then there’s no tax to pay unless you’re making a profit (i.e. selling for more than you bought it for), which is unlikely for most Vinted sellers selling their old clothes.
As to how HMRC can tell, we have a self assessment tax system so you have to honestly declare it yourself. If you’re lying or misrepresenting the position, that’s illegal (tax evasion/fraud).
If HMRC disagree with your analysis they’ll ask for records to justify your position. If you have evidence these are just old clothes (all sold as worn, only one of each item sold, proof of purchase for any of them to show you held for a long time, pictures of you wearing the clothes), HMRC will accept your position and no tax return or tax is required.