“Mark as paid” used to mark selected ads as payment completed. It works on ads that require payment before being published on website.
On Classibase some categories can be marked as paid category, for which users have to pay specified fee in order to post in those categories. In case user had problems paying with paypal or want to give you money by hand then you can mark those ads as paid using “Mark as paid” button.
It will not make anything on categories that do not require payment to post an ad.