Glass is half-empty: the "For Sale" sub-forum is where I, like many others, learned a ton about cars. Some of the allegedly "irrelevant" chatter that used to take place there was critical to that learning, and from a commercial perspective it brought traffic to the site. I get that there may be a trade-off with discouraging some folks to post ads lest their cars get trashed, but... One really was able to learn about the C-body marketplace, Mopar history, and the cars being sold through those meandering exchanges -- and there was (with one caveat below) no better way to motivate those exchanges than to do so amid the excitement of cars being put up for sale.
What your not considering is how many times over the last 20 years of managing forums that I have had complaints from sellers about the 'meandering conversations' and/or disparaging comments and those sellers asking those comments to be deleted. Here is a recent one...
The FOR SALE forums are there for the purpose to buy and sell parts and cars. By your explanation, I could theoretically delete all the forums and just have 1 forum where anything goes and that would be just as good.
Glass is half-full: the "Cars for Sale on Other Sites" sub-forum is a truly welcome addition to the site, and I am really glad that you have added it. IMHO, in and of itself it cannot substitute fully for what used to be discussed on the "For Sale" sub-forum -- but the fact that discussions are is still allowed to be much more free-flowing is a welcome feature.
The reason we allow it there is that the seller isn't the one posting the ad and isn't here to complain - so you guys are free to criticize, praise, discuss, etc without impacting the seller directly.