From the Super Mario Wiki
“If there's anything you want to buy, stand in front of it and press .”
Igor is the Boo that runs the shop in Boo's Mansion in 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".