its a novel idea by concept but the headache that will bring forth will not be worth it....like the old adage goes "Caveat Emptor". if the purchaser is willing to go the distance to make a deal..then its all up to him to make sure he is getting a legit deal. This is why is always a good bet to meet someone halfway on doing deals. that way the most you will incur in cost is whatever the amount of gas you spent. Look at the situation not like the glass is half empty..but look at it as if its half full. at least you go out of the house and went for a nice lil cruise
again its not hard to report that seller that made false claims on the condition of said vehicle. there is that "report abuse" button on everyones ad. you can mark them as a "Fraudalent Seller" then the site mods will do the rest.
now having that un-deletable comment box on everyone's ad is like opening up Pandora's box so to speak. granted it will provide comedic relief to an extent...but what will stop that person who is in the line of fire to just delete his ad and repost?