Do you pay stamp duty on a car at auction?

No — stamp duty doesn't apply to buying a vehicle in the UK, whether at auction, from a dealer, or privately. This is a fairly common misconception, likely carried over from stamp duty's association with property purchases, but it simply doesn't apply to vehicle transactions.

What you actually pay on top of the hammer price:

• Buyer's premium — a fee charged by the auction house on top of the winning bid, usually calculated as a percentage of the hammer price (sometimes with a minimum flat fee), and this is the main additional cost most buyers need to budget for
• VAT on the dealer's margin — for vehicles sold under the VAT margin scheme (common for used cars sold by VAT-registered dealers or auction houses), VAT is charged only on the seller's profit margin rather than the full sale price, and this is sometimes itemised separately depending on how the auction structures its fees
• Other administrative fees — some auctions add smaller fees for things like online bidding platforms or documentation, which vary by auction house

The practical takeaway:

There's no stamp duty to worry about, but the hammer price is rarely the full amount you'll actually pay — checking the specific auction house's fee structure (buyer's premium rate, and how VAT is handled) before bidding gives you an accurate total cost, rather than being surprised by the final invoice after winning a lot.