The VAT at auction houses, Captain2, is only applied to the
commission, not to the price of an item itself (unless that item is being sold on behalf of a VAT-registered business).
So, for example, a second hand item sold for £100 plus 15% commission and VAT will actually cost the buyer £100 + £15 + £3 (which is the VAT on £15) = £118.
A VAT-registered trader, who is offering second hand goods for sale (whether directly or through a third party, such an an auction site), is obliged to charge VAT on purchases unless that trader purchased the goods through a VAT margin scheme. The VAT margin scheme rules apply where the trader wouldn't have been able to reclaim VAT on his own purchase, such as when buying from a private individual or from a business that's not VAT-registered.
Confusing, ain't it?
https://blog.shorts.uk.com/vat-on-second-hand-goods