The stance from HMRC is that anything you buy and sell for profit, is subject to VAT, regardless of your turnover. You can register at any turnover below the 90k, but that threshold is there for several reasons, such as they don't have the resources to chase down every private seller, plus companies make a loss within the first 2 years and the absence of tax reporting allows small businesses to grow without the burden of tax demands. The threshold is there to basically say, we'll give you plenty of breathing room to grow a business (that could easily fail) but once you hut 90k turnover, we don't care if you're profitable or not, you are now at large enough a scale that you need to be properly reporting your accounting.
However, I would warn anyone aiming to reach that threshold, that you should already be getting your accounts in order. I registered our business after hitting £65k, knowing that I would hit the £85k threshold (at the time) within 12 months. It took us 9 months to exceed that figure, and we were over £117k by 12 months. Do not underestimate your business growth and subsequent record keeping.
Private sellers under the tax threshold may have some advantages in regard to fewer responsibilities in accounting, but that is largely as they are under the radar. They should be paying tax on the profit, declared as their personal income. However, HMRC are very aware of the increase in side hustlers who are effectively avoiding tax, so are making use of digital services to identify them, hence the introduction of MTD and marketplaces such as eBay who now have to report sales information, and will force you into a business account if you hit certain margins.
If you intend to grow your business long-term, I would encourage you to consider registering soon. You need to get into the habit of properly accounting for all your invoices and transactions (Xero is a great accounting service for this) and you need to understand how being registered can give you a lot of benefits too. I've heard of small business owners are have closed down out of ignorance, rather than learn and adopt the required practices.
It's now all a loss. You gain access to wholesale products; you can avoid paying VAT with some suppliers (eg AliExpress/Alibaba) and VAT you pay with UK suppliers can be claimed back or put against your Tax bill. The MTD system is very good in minimising and properly accounting for your expenses, and only taxing tax on your profits.
Buying used clothes might mean you avoid VAT on them, but you still technically owe VAT on profit from the sale. If you're about to hit the threshold, you need to be evolving into proper HMRC practices if you want to develop a larger and more serious business.
