It's too bad we have to have as many rules as we do, because it can leave people hesitant to read them all and harder to remember them all. But the rules have evolved to best serve the membership.
Most of the rules are based on common sense and courtesy, but the rule about starting threads in the Marketplace isn't common sense, so you'd never know it unless you read the rules and notices. We have that rule because it helps improve the quality of transactions in the Marketplace forum. It's an inconvenience to well-meaning new forum members, but a benefit to most members overall.