By default, Classipress pricing model includes charging a percentage on the sellers ad price of which might be what you are looking for. However charging is upon submission of ads and not during the consummation of sale.
Classipress already includes bank transfer as one of the payment gateways.
To sum it, your theme requirements will require further code customization. The theme is not encrypted and so it can be modified accordingly. Implementation is just dependent upon your coding skills or you can avail the services of an experienced coder to do it for you. You can also contact some of Appthemes Certified Partners
http://www.appthemes.com/partners/ for customization jobs.