1. You can disable registrations and place listings yourself if you wish, however if you still want to allow registration and also charge for that it would require customization on your part. AppThemes doesn't offer customization services, however we do have a
certified partners page where you can find developers for hire.
2. No. Users are charged to place listings, but as you have indicated you don't want them to do this, it would have to tie in with your modifications.
3. PayPal is the default payment gateway, however you can also find
other payment gateway plugins in the AppThemes marketplace. Thanks.