At present there are several on-line bookstores (OLBs) which, for a fee, enable you to display your inventory and Wants, making them available for browsing by millions of potential customers on the Internet. When a customer finds an item of yours they wish to buy they contact you and negotiate the sale with you. This in effect gives you access to a world wide customer base. OLBs also post details about books you are looking for, Wants, so you can use them for getting hold of those hard to find items. See here for more.