Read: http://www.computerforum.com/computer-equipment-sale/announcements.html
You have to post a price and picture with the picture having a slip of paper with your user name and CF on it. I seriously doubt you will sell a computer you build on here, unless you are going to part it out. You might have luck on craigslist, ebay, or very doubtful, but hardforum.com if you want to make a profit.