Despite being fairly new around here I would be leaning on the side of no comments.
I can see the value of commenting on for sale threads for various reasons such as sellers leaving details out so people asking further questions can prod for details people are wanting being the main one, And generally can give you a feel for the seller in that they are forward with giving information, History etc.
But this is out weighed with comments I (and others) have seen. People with no interest in the car for sale just being troublesome, Making snidey comments on price, condition etc and then starting pages of back and forth, Although I havent seen anything malicious as a seller I would be a bit annoyed if my for sale thread turned into pages of useless Jibber Jabber.
If someone wishes to advertise their car for X price then that surely is up to them. Ultimately potential buyers will pay what they are willing to pay. A potential buyer will surely have looked at various for sale sites and got a guage on what prices are for spec, mileage history so will be aware if they are getting a good deal or not. Comments like "Good luck at that price :? " Isnt particularly good for anyone involved.
The other ones I have seen, A car for sale with listed bad points honestly from the seller, Only to be told in the comments he should fix them as it shows the car has not been maintained. Again if a seller wishes to list his car with faults mentioned and reflected in price rather, This is surely upto the seller, Could be numerous reasons for it. Time, Finances, mechanical knowledge.
The worst I seen was someone commented that the photos were bad as it shown the car had been recently washed as there was soap on the floor. I couldnt get my head around that one and would prefer not to have clogging up a for sale thread.
I agree there are times needed for comments but being the internet some people just dont have a moderation of control and say things they would never consider saying in person. Could just be me but I view a car sale as a fairly professional thing and would treat it as such so I would always PM regarding details rather than publicly making comments on threads.
My recent experience purchasing a coupe on saturday from a forum member on here. All dealt with through PM for any details I was looking for and arrangements made from there. Although the car sale was not as professional as it could of been lol, It included taking a tourist trip of leeds trying to find a bank while google maps sent us in circles, Possibly expecting a fine for driving in a bus lane on a test drive and then forgetting signing some paperwork. But he was a Genuinely nice guy, Knew his stuff and in the end was a good sale.