The intent of the classifieds, is just a convenient place to make a friendly exchange. There plenty of places on the web to run a business, or set up shop. I uses the 'New Posts' button, and to open up the classifieds to greater volume, would mean looking through pages of posts, and picking out the Metal Detecting posts. The alternative, would be to go through each forum, and subforum individually, and manually check for new posts, on each thread. It wouldn't be so fun and friendly.
A forum site is mainly for exchanging thoughts, ideas, and experience. We have equipment, that has served us well, and we upgrade. Our old equipment, is often an upgrade to another member, who would appreciate a chance at getting a good deal, from someone who isn't out to recover every penny invested, or pay the bulk of their new equipment. We find things, some keep and collect certain items, others only want to cash it in. It's nice to give a fellow hobbyist a shot at it first, before melting it.
A friendly exchange, or occasional sale of personally owned and used isn't a big deal, but to buy things with the expressed purpose to resell for profit, is a business. Operating a business, complicates things, and brings up tax questions (yeah, need to sit down and file mine soon).
Really shouldn't take a month, to reach 100 posts, and gives you some idea if this site inspires you. It's a great place, who can resist?