How about changing the way the for sale/wanted sections works. So that people can respond to for sale threads like they can in wanted threads.
You could charge the £5 still to cover the cost of the extra moderating.
You could then also have in massive bold red writing, do not finalize your sale via PM, please state who you have bought from/sold to in the thread at all times. Or something similar.
This may even work just in the wanted section where the 'I have what you want' Type messages seem to be most common. Raise awareness to offering goods via PM and keep them on the thread. This would allow mods to spot things earlier and people posting wanted ads to have an in your face style warning?
Just an idea, open to being ripped apart
edit, Incorporating the feedback onto the members profile page may also be a good idea. So people can instantly see/leave any trade feedback by clicking on the name.
I think raising awareness and policing is the way to go myself, a small charge is more than acceptable for it too.