No this is not a feature of ClassiPress, as once the buyer uses the contact form, the seller will see their email address, and once the seller responds to the buyer, they then will see theirs. It would definitely require customization of the default theme on your part if you were looking to implement this. We do not have any existing documentation relating to such a change, but if you were looking for someone to make the modifications, 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.