YOU can be sold in seconds.

No, wait: make that milliseconds.

The odds are that access to you — or at least the online you — is being bought and sold in less than the blink of an eye. On the Web, powerful algorithms are sizing you up, based on myriad data points: what you Google, the sites you visit, the ads you click. Then, in real time, the chance to show you an ad is auctioned to the highest bidder.

Not that you’d know it. These days in the hyperkinetic world of digital advertising, all of this happens automatically, and imperceptibly, to most consumers.

Ever wonder why that same ad for a car or a couch keeps popping up on your screen? Nearly always, the answer is real-time bidding, an electronic trading system that sells ad space on the Web pages people visit at the very moment they are visiting them. Think of these systems as a sort of Nasdaq stock market, only trading in audiences for online ads. Millions of bids flood in every second. And those bids — essentially what your eyeballs are worth to advertisers — could determine whether you see an ad for, say, a new Lexus or a used Ford, for sneakers or a popcorn maker.

One big player in this space is the Rubicon Project. Never heard of it? Consider this: Rubicon, based in Los Angeles, has actually eclipsed Google in one crucial area — the percentage of Internet users in the United States reached by display ads sold through its platform, according to comScore, a digital analytics company.