Can anyone tell me how to start a new topic in the marketplace section? the site isn't allowing me to start a post and idk why..

 

You need to be a member of the site to sell any items on this site. Even though you are registered to the site, you are still considered a guest until you pay for a membership

Click on your profile at the top right, there should be an option to purchase a membership

 

You should also read the rules. They cover this.
https://www.s2ki.com/s2000/topic/457...fore-you-post/