There's more to the posting requirements than just a means to do a "background check", so to speak. I would imagine people who run and pay for a site don't want everyone who wants to sell a car, but has never contributed to their site, to sign up just (or mostly) for that purpose. You're basically using the site for free advertising/exposure for yourself without giving back to the site or community. I would hope you can see why that might be frowned upon.

Oh, BTW, if you think people on craigslist are "cheap", wait till you see the offers you'll get here. The fact that people in a car specific community know that car pretty well can sometimes backfire on you because it also means they know when you're asking too much.