Here's another idea.....
What about giving the seller the option to allow comments on their ad?
If I post something, I'd always allow comments. It would save me getting 20 DMs because I forgot to list a seat post diameter, or having to reply to 10 messages asking if something is available with "kind of, you're 3rd in line" or whatever. And if I'm selling something, it would actually benefit me as a seller to have guys commenting stuff like "awesome bike, I wish it was a large". I would also always sell something to whoever commented "dibs" first 😉.
@ACE Cycles, apart from what Hairy mentioned, it's also an opportunity for sellers to create some "good vibes" without having to post on the forums. For example, if you post a few ads, and I can see that you respond to comments, and are courteous and helpful etc and that there aren't any "this guy screwed me over" comments, I'd feel more comfortable in trusting you without just relying on your ratings, or your forum post count. For the buyer that isn't engaged on the forums, it can really help to build some confidence in sellers if you see some engagement under the ad. I bought my first bike off the classifieds way back when, and was clueless about bicycle and the forums, but I saw in the comments under an ad that the guy seemed to be a decent human being, and saw from the comments that the bike seemed to be a good deal, and pulled the trigger.
You might say the rating system should serve this purpose, and I guess it does in part. But technically there should only be reviews for concluded deals, so non-responsive sellers don't get rated, guys that respond to "the year model on your ad is wrong" with abuse don't get rated, guys that beg and harass you for lower prices don't get rated, guys that consistently low ball you don't get rated, etc etc etc.... And also, if I were to open an ad, see that 5 guys have asked questions and the seller hasn't responded in weeks, I'd know to just ignore that ad without becoming the 6th guy to just get ignored. If you look at half of the "I'm upset about the classifieds" threads that people start, lots of those threads wouldn't exist if there were ads on the classifieds (like the "why don't sellers respond to my messages thread", or the "is it still available" thread)