We don't have any mods that are gung ho about passing out infractions. You have no infractions. Your sources are incorrect.If I may use another website as an example, take a look at the S&W Forum. That site has national exposure with quite a bit more traffic than you have here. Their classified section is pretty straight forward, you need a pic, description, etc and are allowed to bump once every 24 hrs. People can comment and the seller can reply to comments as they arise. No outside discussion is allowed, no comments either negative or positive. No "great gun" no "your prices are too high" etc. If someone makes such a comment a moderator will remove the comment and give the party an infraction. Too many infractions and you get a "time out"
Since you already have a mod or two that are pretty gung ho about passing out infractions then moderation should be quite simple. You allow comments yet keep everyone in check. A pretty simple solution if you ask me.
As for your idea...so you are proposing that comments ONLY be allowed for asking questions? Nothing else? No discussion on prices, etc? I don't see how that is a benefit over PMs besides the seller getting less PMs. Oh, and the seller then has to check his thread often to see is people have questions. PMs email you a notification and pop up on the board. Also, I would hope that sellers are making descriptive enough ads that they don't get tons of PMs with questions.