Building a customer database is so critical to any type of business, yet businesses don't show a lot of creativity in how to build those databases. Building a database of people who've bought from you, even more critical.
Australian business, Entertainment House, is one of the top sellers of DVDs and CDs on ebay. It only costs them a few dollars per sale - yet once people have bought from them, they become an Entertainment House customer and not just an ebay customer. Entertainment House can then legitimately market to those customer's who've opted in to hear more for them.
So they've made a sale, added a customer to their database - all for a few dollars, which they make back in profit from the sale. A perfect zero budget marketing technique.
Could you sell anything on ebay? Even if you're a service, you'd be surprised what you can do. Take the guy selling half price "paintball" sessions. Once people get there, they usually play more, they make additional margin and they've overcome the biggest hurdle most businesses face - getting them to buy the first time. Go on, get a bit creative...