Yeah, see here: https://ananostr.com/

As another commenter mentioned NIP 99 has already created a custom event for classifieds its really up to a client to support it or the standard clients and relays to start reading and supporting it and you could see classifieds not only on a specialise site but on any nostr site that will show those events

There's a NIP for that: https://github.com/nostr-protocol/nips/blob/master/99.md

I have no idea if anyone is using it, but it wouldn't be difficult to get info on it's usage. Just check a bunch of relays for kind:30402 notes.

craiglist has a lot of functionalities... what exactly are you looking for?

possible, but doesn't exist as far as i know

Check and try the ~AGORA territory