Stock from outside EU . Hammer @ 5.5%, Premium @ 20.6%. On proof of export the VAT on the hammer can be repaid by Christies.
The VAT on the premium cannot (place of supply in France and not covered by the reverse charge procedure)
As a note, if the buyer could prove that the item was purchased as a business expense, the buyer could apply directly to the French tax authorities for a refund of the VAT on the premium ( 13th Directive refund).