Yes I'd actually prefer a forum-style classifieds section so that questions and answers can be provided to everyone and also so things can become more "public record". It might also save the seller from responding to 8723 emails asking the same question about the thing he's selling.
The problem is, I still think that it should require the same fields that aBay currently has, like Condition, Price, Location, etc etc. Basically it'd be the current aBay, but with the capacity for leaving messages. Classified sections that are essentially just normal forum format tend to suck.

I know this would require more work, but IMHO it'd be the best format.