Viral marketing is a technique for spreading product or service information the same way a nasty flu virus is spread. One person spreads it to another, the second person spreads it to three others, and so on.

E-mail is the main tool that makes viral marketing work. Someone happens across your Web site or sees an item you are auctioning online and forwards your Web address to a friend. That friend has several other friends who might be interested in what you are selling. So, she forwards your URL to them. One of the friends has a cousin overseas who has been looking for an item just like what you are selling. He contacts the cousin; the cousin isn't interested, but his mother is, and she e-mails three of her friends with the news. Suddenly, you start getting inquiries and bids from all over the place.

The ubiquitous E-mail to a Friend link on eBay's auction pages is a prime example of viral marketing. Imagine Joe in Buffalo checking eBay with his wireless laptop computer while enjoying an after-work cappuccino. He comes across an auction item that amuses him or brings up a specific memory. He quickly dashes out a message to a friend: “Hey, remember that really ugly bike you had when we were kids? This may be it — or one just like it.” As he sips his drink, another memory pops up. Someone at work told him recently that the in-dash radio had died in her lovingly restored 1957 Chevrolet and finding another one was going to be a pain. Joe decides to do a Web search — and finds the one you have just put up for sale. Zip, zip, the news goes out, and another potential buyer shows up, thanks to viral marketing.

Unfortunately, viral marketing only succeeds on a larger scale if the pass-along rate stays high for a while. If only one person needs the in-dash radio for a '57 Chevy, the “virus” quickly dies. However, if you are selling something outrageous that has just been in the news, such as the ball that hit a batter and sparked a two-hour riot at Fenway Park, word may spread quickly and widely. Hundreds, even thousands of sports fans may e-mail each other with the news, and then start clicking on your Web store to see the infamous baseball.

Only one of them can buy it, of course. But chances are good that at least a few of them will click on some of the other items in your inventory and buy something. Either way, you win.