The Antique Dealer is used to Sell the "Art" gathered from the Art Gallery, you also have the option to buy an Excavator for £1,750. If you have some Art to sell to the dealer you walk over and scroll wheel to access the pad, you then click on the "Modern Art" and click the sell option, each art piece will sell for a grand total of "£250,000". Once you have sold you can go to a ATM and store your money or proceed to spend it as you please. The Antique Dealer is a legal location and is located at "036137" in Kavala.
It is also the Location where you sell items excavated from the shipwrecks.