VAT must be paid on a lot subject to VAT on the hammer price and separately on the buyer's commission. The VAT is a percentage that differs per Member State in the EU. In Belgium it is 21%.
 

Shipments within the EU are subject to tax.

• Business buyers can reclaim the payment of VAT in Belgium and for shipments to destinations outside Belgium there is a deferral (reverse charge) of payment.
• Private buyers pay the VAT of the destination country.

Lots subject to VAT are completely free of VAT for shipments outside the EU / to a bonded warehouse, if Sylvie's receives the legitimate proof of export.

Example

If the buyer's commission ex VAT is 20% over the hammer price, and the VAT would be 21%,  the calculation is as follows:

At a hammer price of €100.00, you pay 100*1.21 + (20%*100)*1.21 buyer's commission. Together that makes  €121.00 + €24.20 = €145.20