I'm looking for a way to charge users to post to a directory site.
I've tried using the WPMU Dev directory plugin, but it didn't have this feature.
You could either charge a one time fee per post, or you could charge a monthly rate.
Oh, and for now the gateway would be PayPal.
Cheers.
There are a number of ways you could do that on the PayPal platform, but with the limited info I have here I would probably recommend the Adaptive Payments API, specifically Preapproval and Pay.
Using Preapproval, you would have users create a preapproval profile which gives you authentication to make payments on their behalf without further approval. This way you could take payments based on any trigger you want, be it a post, a specific date coming up, or whatever.