Instructions from the Internal Revenue Service make clear that all income can be taxed.
More than 135 million people have registered to use eBay, the auction site that calls itself "the world's online marketplace." Buyers bought more than $34 billion worth of merchandise there last year.
The IRS can apply a list of indicators that might prove whether someone's online auctions amount to a business. EBay does not report individual sales to the tax authorities, but urges users, in the site's educational materials and seminars, to learn about tax issues.