From the Super Mario Wiki
The Information Booth is a store in Beanbean Castle Town that specializes in selling information. The shopkeeper sells "information" to Mario and Luigi for 100 coins, and this information usually revolves around their current objective and either where it is located or how is can be accomplished. His information only relates to the main story and nothing else. Near the end of the game, when Mario and Luigi are ready to go to Bowser's Castle, the Information Booth has no more information to give.