I figured this would happen sooner or later. I am very pleased to see a thoughtful approach being taken and one that won't be abused by people flying in here to sell their junk (or stolen property, in some cases) and messing with the community instead of adding to it. The idea you are pursuing is excellent. Charging points will persuade people form posting indidual items in separate posts too, which will keep clutter down. I would add that you should not permit threads of complaints by new people showing up trying to sell or buy hoping to circumvent or change your rules. They are a waste of space and time. There has been constant whining over at AGN, just about a simple ten post rule, before using the classifieds.
I think the points charged should probably be dynamic and not fixed. There are a lot of very good sellers that buy and try and sell a lot. They are a boon to the community and a great resource. So maybe after three or five great sales, with both buyer and seller happy, you get your points back. There is no sense to be charging people for doing us all a service. A minimum of 100 points to post and I would personally like to see it last a month. Why? Because often I don't really need something and I tell myself that if someone really wants it they can have it, but if it's still there after a while I'll buy it. But two weeks could work too.
And by dynamic, I mean that after multiple postings and multiple successful times of recouping your points, say three times, it only costs you 50 points to post. All leading to a successful 100 sales, or whatever, you are no longer charged points at all.