I thought I'd share some experience from other forums.
A key to minimize modwork is to restrict the ability to comment in threads. That saves tons of trash talking and problems. Each thread is basically just a sticker presenting the deal, only OP can post in thread ...and the rest is done via PM at the discretion of users.
The key here is to only let OP have access to each thread.
Setting up this type of classfields requires quite little moderation other than removing spam and an initial 'disclaimer' where the 'rules are laid out'. (sort of - we don't assume responsibility, use at own risk, precaution XYZ etc, don't bump too often etc). Then it runs on its own.
But - frauds? trolls?
Another added feature is a special 'sales reputation'. That's a bonus.
Ie, buyers and sellers get the opportunity to rate each other. Even if reputation is on the line for the guys with lots of posts... ..posts, forum charisma or skills are not indicators of business trustworthiness - is my bitter experience.
I've seen reputations-lists been done on user initiative, as in adding 'successful transfers' to a list maintained by each user as "references" to at least have some sort of credibility check.. That has been really helpful, even in it's simplest form.
On another forum where I've been selling and buying stuff for a couple of years is literary a sailing ghost ship without any signs of a moderator for a few years.
The part of the forum that does best is is the classfields. That said, I fail to see why it would be much different here.
Cheers.