payment method procedure and gateway for a website

I am making a website for a person who's provoiding a plateform for sellers and buyers. Actually there are three users of website (seller, buyer, and middle man). seller will list the products, buyer will buy some product and middle man will get some percent of the price of product. making this with php.

I have some confusions about payment process. for example if buyer buys a $50 product, he pays $50 from which 5% will goes to middle man's account and the rest will goes to seller's account. How can I do this?? I want this whole process to be done automatically through credit card.

Note: The website is for Pakistan only, so buyer can pay from easypaisa or jazzcash also. And what paymnet gateway should I use and how. I need some detailed answers please.