People can share their market place listing with their social circle. That includes specialized groups, but also friends.

I recently sold a 35 year old camper van, with one click I shared it in a group for old cars. People who saw it in turn shared that again in other groups for cars.

Users see those listings in their feed, on the same page where they might see news or funny dog videos.

If I sell on my local version of Craigslist, only people who actively search for a camper van would see it.