How does the margin scheme work?
Second-hand goods are usually bought from private individuals, who cannot charge VAT. Under normal rules the dealer would then have to charge VAT on the whole selling price with nothing to reclaim, taxing the same item twice over its life. The margin scheme fixes that by taxing only the dealer's margin.
The VAT is worked out as the VAT fraction of the margin — at the 20% standard rate, one sixth of the difference between purchase price and selling price. It is not shown separately on the invoice, and the buyer cannot reclaim it. If an item is sold at a loss, there is no VAT to pay on it, and the loss cannot be set against the margin on another item unless the business uses the global accounting variant.
The scheme is optional, item by item. Each item must be recorded in a stock book with its purchase and sale details, and there are specific rules about what the purchase and sales invoices must say. Lose the records and HMRC will charge VAT on the full selling price instead.