Techniques are described for facilitating transactions involving items and users in various ways. In some situations, item transactions are coordinated by an automated Item Transaction (or “IT”) system provided via one or more computing systems, such that users of the IT system who have available items are matched with other users of the IT system who desire those items. The types of items being involved in transactions via the IT system may vary, and in some situations may include items such as music CDs, video DVDs, computer games, computer software, etc. The operation of the IT system may be enhanced in various ways via interactions with one or more external item marketplaces, such as by automatically obtaining and using information related to users of the IT system from those marketplaces, and/or by acquiring additional items from those marketplaces for use by the IT system.