From the Super Mario Wiki
Igor is the Boo that runs the shop in Boo's Mansion during the events of Paper Mario. When Mario first enters the shop, Igor will refuse to sell him anything under the orders of Lady Bow. Once Lady Bow has joined Mario's party, Igor's shop is open for Mario. There is also a letter that Mario has to deliver to Igor for a Star Piece. Goombario's tattle for the Boo states: "He probably was a merchant before he became a Boo".