No, I'm sorry, I can't give you any answers specific to Pakistan. Only someone who is knowledgeable on Pakistani law can do that.
But it's safe to assume that you will be subject to strict regulation in Pakistan, just as you would in most other countries. After all, you are proposing to hold money on behalf of other people. In that respect, you will be acting similarly to a bank (albeit on a much smaller scale). So it's likely that you will be subject to the same controls and regulatory regime as a bank, and these could be considerable.
Leaving aside the legal aspects, I wonder how you can make a business case for your proposed service. Have you done any market research to find out if people would actually use an escrow service when buying good on line? Are you sure you can convince people to trust you to hold their money?
Also, have you any experience of this kind of service? The fact that you have to ask these basic questions suggests that you haven't. That in itself doesn't mean that you shouldn't do it, but it does highlight the need to take proper professional advice (which you must be prepared to pay for).