If I understand your problem correctly, you're attempting to use the API on behalf of someone else. For my business, I am providing a marketplace where sellers can sell physical products to buyers. In order to do this, I need each and every seller to give me permission to create orders, capture payments, and give me transaction data on their behalf. This is done by onboarding sellers. My use case specifically requires them to sign up before selling products, but there's a way to do it after a product is sold. I am not as familiar with the latter use case.
https://developer.paypal.com/docs/platforms/
https://developer.paypal.com/docs/platforms/seller-onboarding/before-payment/
You call the partner referral API and pass in the permissions you want and have them sign up through PayPal. Once they accept, PayPal will pass back a merchant id that you can then use to create orders on their behalf.
If this doesn't work for you, the seller can manually grant permissions using their account. They have to log in to PayPal, go to Account Settings, select Account Access under Account & Security, click the Update link next to API Access, and then specifically grant access to you based on the name of your business. I find this less than ideal, but maybe it is what you need.
