How do I obtain permission to process on behalf of a PayPal merchant using the PayPal REST API?

Question

I would like my website to be able to process PayPal payments on behalf of merchants. The merchants will have their own PayPal accounts and will need to give me permission to do so.

Permissions API Deprecated

PayPal have a "Permissions API" which allows a third-party such as a hosted shopping cart to process on behalf of PayPal merchants. See: https://developer.paypal.com/docs/archive/permissions-service/integration-guide/PermissionsUsing/

The above page says:

Adaptive Accounts and Adaptive Payments are no longer available for new integrations. PayPal provides documentation for these APIs to support existing integrations.

PayPal REST API

Are these things possible using the PayPal REST API:

• perform a handoff from my site to PayPal, have the merchant grant my site permissions to act on their behalf,
• process payments on behalf of the merchant using the credentials obtained from the first step

Answer 1

It's is possible to use the PayPal "Marketplaces and Platforms" API to achieve this:

https://developer.paypal.com/docs/platforms/seller-onboarding/

Answer 2

Every merchant can obtain ClientID/Secret credentials from the Live tab of a REST API App via https://www.paypal.com/signin?intent=developer&returnUri=https%3A%2F%2Fdeveloper.paypal.com%2Fdeveloper%2Fapplications , and enter them into their config interface on your platform.