Why not do as a lot of other forums do, premium membership of say £10 a year and they are the only ones who can sell stuff plus on another forum I am on you need 1000 posts before you can sell something.
Charging to sell will reflect in a negative way. As I've said people come here purely to sell, and a lot of the time it might just be once or twice, why would someone pay a tenner to sell on here? It would be cheaper for them to go to eBay.
The post count theory won't work either as has already been mentioned. If someone wants to make a few hundred from scamming, it's not going to hurt them much to stick around for a few weeks and post to get their count up.
I think Guy's feedback idea would be a great idea. At least it would provide users with a more trusted view of the seller, regardless of posts. People that only come here to sell then will be able to build a good foundation based on feedback, and long running members also will have a chance to do it as well. No doubt there may be a couple of times someone has sold, got good feedback and then scams, but it would most likely be very very rare occasions.