Build website with login and password and posting ability

I am interested in a simple web site where people can post items for donation, buy/sell, trade (i'll specify what items these are later in the process).? It is similar to craigslist but in a specific niche. top center of the web site:? A statement saying something like "Buy/Sell, Trade, or Donate _____" Directly under that it says "Seach ______site" with search box underneath Under the search box it is the most recent listings going down the page Upper right corner:? "Post to _____site" I intend to monetize the site through advertisements so I want big spaces on the left and right side of the list so I can ads, banners, etc.? Also would be good if when people click on the item postings and go to that page they can see advertisements, banners, monetization methods, etc. To post to the site, people must be able to create an account with their email and password, and then have their email be anonymous.? And then I guess I would need a message saying when they reply back to someone their message will now show their email (but not post their message on the board) this would just be an email between the two people (just like craigslist).? I don't know if? I need the post to expire after 30 days but worth mentioning for thought. A couple other things I don't know if I would need are "terms of use, privacy, legal, faq, etc. Also, what do you do to keep people from posting unrelated things on the list ie: porn sites, swearing, stuff like that.? Or do I have to constantly monitor and delete posts???