From the Craiglist site:

craigslist sites are based on geographic location

This is a map and list of our sites:

https://www.craigslist.org/about/sites

You can find the closest craigslist site for your geographic location by zooming in on the map or browsing the list underneath the map.

Please note that the site location for a posting cannot be changed.

You will need to start your posting over again from the beginning if you have chosen the wrong site.

It is not possible to post to more than one site at a time.

This sounds exactly how Flohmarkt works, exept that they have a convinient map for you to find the location specific site. This would probably be a nice feature for Flohmarkt as well as part of an instance chooser.

I guess what you are stuck up on is browsing listings, not posting them. But that is more like an intra-instance search tool similar to how there is the Lemmyverse search engine for Lemmy. But I find that of very limited use for a location specific market place like Flohmarkt, where you already know which location you are interested in and don't need a search engine or drop down box to confuse you with listing options from entirely different locations.

I think there is some talk about adding more fine grained location specific groups to Flohmarkt (a bit like Lemmy communities), that would probably also allow posting from another federated account into them, but that would likely be counter productive as it would dillute the explicit location specific focus.