Well, at least in other places of the web such a decentralized marketplace is already a reality?

Check BlockAze for example. It encodes all listings and market acticity fully on the Bitcoin blockchain and is active right now.

Here is its source repository:

https://git.tchncs.de/BorealUS/blockaze

You can find tutorials on YouTube or its full manual and code in the above link. It currently forgoes escrow in favour of a trust/ranking system.

I think it fulfills almost all criteria here and is actively used / maintained. Due to its emerging popularity it's API has been
added on https://moofetch.mooo.com, next to other established tools like Fulcrum SPV.