Have looked at the reputation type stuff before but I don't think you can easily tie it down to classifieds sales that someone has made. You could just +1 style feedback anyone.
Unlike what someone recently said, I have put a fair amount of research into this. Where I tend to end up stuck is that the scamming is so so far in the minority of trades that you are putting in so much work to try and get rid of that tiny percentage.
E38's move is an interesting one and already it seems that an increased amount of semi-pro traders have appeared on here
One move I keep coming back to is moving the classifieds out to a totally separate bit of software that does have more of the features we might want - templates, reputation etc. Whilst it may mean everyone having to register a new account it might work well in the longer term.