The threshold is on gross sales, not what you receive
Amazon, eBay and Etsy deduct fees and settle a net payout. For VAT purposes, <a href="https://www.gov.uk/vat-registration">taxable turnover is the gross selling price</a> before the platform takes its cut. Sellers who monitor their bank balance or settlement payouts against the £90,000 threshold will breach it without realising, because the threshold is on gross sales.