The taxable amount in respect of each supply of goods referred to in this Section shall be the total amount invoiced in accordance with Article 339 to the purchaser by the organiser of the sale by public auction, less the following:

(a) 

the net amount paid or to be paid by the organiser of the sale by public auction to his principal, as determined in accordance with Article 337;

(b) 

the amount of the VAT payable by the organiser of the sale by public auction in respect of his supply.