For example if CDNow knows that you liked the Phantom Menace CD it can present you with a list of other CD's that customers who have bought the Phantom Menace CD have purchased in the past. Once again, this is something that would be impossible offline. Thirdly they provide detailed information, sound clips and Reviews on the hundreds of thousands of titles. Visitors can not only learn about albums that they might be interested in purchasing, but they can read what Rolling Stones had to say about it, and listen to a few 30 second sound clips from the album. A feat like this would be impossible.
Buyers get an enormous categorized selection of items that they can choose their price on.
Sellers get exposure to an enormous audience of buyers, and a chance to set minimum bids so that they don't lose money. We can all learn from the two exmples above. Try to think about ways that you can make your site more then just a web brochure. Taking secure online orders is an easy first step. If you can find ways to do things on your site that help the customer that would be impossible to pull off in the real-world your website will be onto a very good start.