Please don’t post in the Alpha/Beta forum unless you’re actually testing the alpha/beta version of WordPress (not a plugin or a theme). Also, once WordPress has released a version, it’s not appropriate to post in alpha/beta and you should post in the How To & Troublshooting forum:
https://wordpress.org/support/forum/how-to-and-troubleshooting
This post has been moved out for you.
Your question is really too general to be answered in these forums. People have, literally, written entire books about these very subjects.
For what it’s worth, there are a lot of e-commerce plugins out there that will turn your WordPress site into an online marketplace. Download some of the free ones:
https://wordpress.org/plugins/search.php?q=ecommerce
and play around with them to figure out how they work and which one is probably going to work best for you. Don’t spend money on a commercial plugin until you have a better idea of what you want to do and how to make WordPress do it. (A lot of people run successful online shops just using plugins from wordpress.org.)
