Facebook Inc. (FB) is getting serious about monetizing Marketplace.

Twenty months after introducing its Craigslist-esque portal for local goods and services, Facebook is letting anyone buy ads peddling anything from used cars and plumbing services to your neighbor's extra bottles of mayo.

In a blog post earlier this week, Facebook said that advertisers, including both brands and individual sellers, can now pay for ad placements in Marketplace in a handful of countries.

"Over the next few weeks, all advertisers targeting audiences in the U.S., Canada, Australia and New Zealand will be able to run ads in Marketplace using the traffic, conversions, product catalog, video views and reach objectives," Facebook said in the release.

If you've ever checked out Marketplace, you may have noticed that it contains a pretty eclectic mix of inventory. From unwanted foodstuffs and clothing to used furniture, cars, and even apartment rentals, anything goes on Marketplace, for a price of anywhere from $3 to thousands.

It's not clear how many people are actively buying and selling on Marketplace, but Facebook CEO Mark Zuckerberg said at the company's F8 developer conference in April that it's visited by 800 million people around the world every month.

It's a potentially big opportunity for Facebook to capture local classifieds, an area still dominated by stalwarts such as Craigslist and newer apps like Letgo and OfferUp. The social networking giant has tinkered with the portal over the past several months, introducing segments such as used cars and home services through promotional deals with dealerships and booking companies like HomeAdvisor.

"It's a multi-billion dollar opportunity, and unlike most websites that are in the marketplace and classifieds field, they don't need direct revenue for it to make it work," says Peter M. Zollman of the Classified Intelligence Report.

The listings you see on Facebook's Marketplace may be less about selling anything than simply getting your attention, Zollman explains.

"If you look at Facebook, they rely more than anything else on user engagement. Because they learn that much more about you, and because people looking to buy or sell can drive so much increased engagement, they can operate without selling a single penny's worth of ads."

That could also explain the dubious appeal of some of the items that may show up in your Marketplace section: Whatever captures your attention, whether positive or negative, translates into useful data to fold back into their core advertising business.

"They may be spraying people with stuff to see what transpires," Zollman adds. "They're not stupid when it comes to targeting."

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