If you are trying to build a marketplace maybe it is better to search for another open-source solution I’m no expert here, so you’ll have to do your research based on your needs. Take a look at Sharetribe.
If you want to do it with Discourse you’ll have to invest a lot of time in customizing CSS, and maybe custom logic with JavaScript and Ruby.
Also for data-fields like $Price search here on meta for “custom fields”
(just in case) you can hire someone like here:
For your other questions (not directly related to this plugin) creating a new topic might be better. 