Not a bad idea to make it known that any comment/s about transactions conducted via the forum, may be listed for others to see. It already happens but if it is there in black and white that any deal may be subject to comment on the forum it can't hurt just as long as it doesn't turn into a 5 page slanging match. Perhaps any comments on the deals done should be kept between the buyer and seller only to avoid any friends or foes putting their 10c worth in. That way it should be kept short and factual, no hearsay or bias.
Alternatively, could a feedback system, similar to that on eBay, be incorporated into the forum software?
End of the day, you got nothing to hide - you got nothing to worry about.