a webforum. probably using phpBB. BUT, what should I do it on? I want the forum to have a buy, sell and trade forum because it obviously attracts people as it's somewhere for them to advertise there services for free... But I don't just want another webmasters forum.. Any ideas? What do you want to see?
Best thing to do is find a theme. I don't mean a style, but something specific your forum will be based on. You can set one up for only buying selling and trading. But remember, doing so you must compete with amazon, ebay and the such. This is easily discouraging but not impossible. Find a bunch of friends that will help you out and fill the forum. No one wants to register to an empty forum. So it will be up to you to get things started. For something like that you have to ask yourself a few questions, Do I have stuff to sell or trade? Where is it possible for me to ship to? Do I have the money to start this? Doing that is actually considered a buisiness in most places. And starting a buisiness takes a lot of thought and effort. Have a look around at other places that do this and get a few ideas. Don't actually steal any ideas from them though Also you will need to get thoroughly familiar with your software. Nothing worse than not knowing what features do what. And being someone that uses phpBB3, I do know how confusing that software can be to beginners. (not saying that I know you are a beginner) Wish you the best.