I find irony irresistible. That's why I'm writing this Take. In the shadow of so-called "Cyber Monday" -- otherwise known as the biggest online shopping day of the year -- there's new research that says one in six Americans sells stuff online.

The Pew Internet & American Life Project conducted the study. Its national phone survey found that 17% of Web-using adults have sold something online and that 2% sell something on any given day. Those numbers could get a lot bigger. In an interview with TechWeb, study co-author Amanda Lenhart said that 35 million, or 24% of us, have participated in an online auction -- either as a buyer or as a seller. And about 32 million, or 22%, have used online classifieds in some fashion.

Talk about a boon for eBay (NASDAQ:EBAY) and Overstock (NASDAQ:OSTK), right? Not exactly. Citing data from researcher comScore MediaMetrix, Pew says that Craigslist, a full-fledged member of our Rule Breaker Universe (access available with a risk-free trial), leads in online classified advertising. Moreover, netizens are increasingly flocking to specialist sites for help. Cars.com, for example, had 3.7 million unique visitors during September, according to MediaMetrix. That's especially interesting because both eBay and Overstock have reported higher customer acquisition costs in the recent past. Maybe these sites have provided more competition than any of us bargained for? It sure seems so.

What's next? That's hard to say, but one thing's for sure: E-commerce is no longer just traditional retail brought online. Everything is up for grabs. Even barter is doable. Craigslist made sure of that. That means the entire retail category is ripe for the kind of sea change that could unleash new Rule Breakers with tidal force -- or allow new classifieds competitor Google (NASDAQ:GOOG) to surf its way to an outsized share of the burgeoning market. Either way, the waves have been big, and it's likely they won't get smaller.

Are you a rebel investor? Get more Rule Breaking Foolishness right here:

High tech. Biotech. Nanontech. Any tech. David Gardner and his merry band of Fools cover it all for subscribers to Motley Fool Rule Breakers , and they're walloping the market by more than 15% as of this writing. Get in on the action by taking a risk-free 30-day trial today. All you have to lose is the prospect of better returns.

eBay is a Motley Fool Stock Advisor pick. Overstock is a Motley Fool Rule Breakers selection.

Fool contributor Tim Beyers sees about six people in the Starbucks he's at right now, which means he's the only one in the group who's sold stuff online. Right? Tim didn't own shares in any of the companies mentioned in this story at the time of publication. You can find out what is in his portfolio by checking Tim's Fool profile. The Motley Fool has an ironclad disclosure policy.