Once I am already logged in on the site, why is it that I am asked to log in as a new user if I want to make a post in the marketplace? Is there a different user process for the marketplace?
Yep. Unfortunately the Marketplace, the Resources and the forum are on different systems. Eventually they will be integrated, but for the time being you have to register/login separately for each.
For the future merge, I would suggest you use the same username and password for each.
Sorry for the inconvenience!