https://www.theverge.com/2018/6/6/17434602/facebook-marketplace-boosted-post-ad-sale

Facebook announced today that it’s testing the ability for Marketplace users to promote their listings to their neighbors. Anyone who isn’t directly in Marketplace will see these ads as “sponsored” posts in their News Feed. Facebook says it’s monitoring the small, US-based test for now and doesn’t immediately have plans for a wide rollout. Users will be able to set their budget and determine how long they want their campaign to last. The ads will be pulled immediately once an item is sold.

There isn’t a set price for an ad. Instead, all submitted ads are entered into Facebook’s ad auction where they will be considered alongside other ads trying to target the same people in the same area. The auction doesn’t reward the highest bidder with an ad but attempts to instead figure out what ad delivers the highest value to both the user trying to advertise and the ad viewer.