ClassiPress does not have any built in functionality which charges after an item is sold, as the sale itself is not carried out within the theme. It would require customization of the default theme on your part if you were looking to implement this. ClassiPress is not encrypted, so you are free to modify it if you wish.
If you are not looking to make these changes yourself, AppThemes does not offer customization services, however we do have a
certified partners page where you can find developers for hire. If you make contact directly with an outline of your requirements, they can provide a quote for the work. Thanks.