GebaN the Broker is a champion wraith. Once beaten, you can speak to him and he opens up shop. Simply buy or sell anything for the achievement. (He's always carrying a legendary weapon, which you'll need for another achievement.)
He can be found in the northwest corner of Cemetery Grounds (follow the wall up from Mausoleum of Bones, he's on an elevated platform with some sentinels), and in the northern area of the Merchant Quarter, just outside of the exit from The Cessways.