To reach that area, you must first travel to Baldur's Gate Southwest then head north.
He is willing to buy all ankheg shells the party has in their inventory for 250. If you have two or more shells, make sure to have only one in inventory to maximize the amount of gold which you could gain. He also refers to Gerde, who can be found in Area North of the Friendly Arm.
After you have filled his inventory (he has 16 slots like any other NPC) with ankheg shells selling them to him one by one and at the same time, he will keep buying them, but will also drop them to the ground. Take the dropped shell, talk to him again, and rinse and repeat for infinite gold.