Ive been buying and selling on internet boards for well over a decade, on probably 20+ different sites, and comments almost always cause more problems than they're worth. Primarily, and this has been mentioned, the problem is tire-kickers b**ching & moaning about the price. Its one thing to think a price is out of reason, but debating it within the ad itself is not the place. Now, if people were intelligent enough to mind their own business and let capitalism function untethered, then it wouldn't be an issue. But, you always have folks who think a price is too high, and are damned determined to tell anyone and everyone who will listen. Once again, doing this in an ad itself is not the place.
Another issue you see is someone may post a unique or otherwise interesting item, and then you have 15 people commenting on how "cool" it is. Before you know it, its a 2 page thread with 15 different people talking about how they had one 10 years ago, or always wanted to buy one, etc. On a site where each response bumps the thread to the top, its a big problem because it moves all other adds that much farther down. Just like with pricing, debating/discussing the merits of an item is not appropriate in a classifieds ad.
Ive also seen issues with people attempting to post offers, devolving the ad into a petty auction. You'll have people post random questions, even when the information is clearly posted in the ad, and others will flat out **** all over an add for little to no reason at all. Removing comments solves this issue, forces people to conduct all discussions through PM/email, and clears the section of unnecessary clutter.
Now, if you had a moderator policing the section, it may help. You could have clear cut rules, and violators could be restricted or banned from the forum for breaking rules...but that takes time, a couple people monitoring the section, and you will always have members crying about getting in trouble. Restrict responses, and its a non issue.