I doubt they will have an API for the Marketplace as it is designed as a person to person local marketplace, rather than a business to person marketplace. However, it would not be hard to build your own bot to post ads using something like nightmare.js. I looked into this a few months back and it took me about 2 hours to create a minimal bot that could post a basic ad. Could share the code with you if you want.
If you’re not familiar with Javascript/Node.js, you could build it using any web scraper but Nightmare.js makes it really easy.