I am going to build an eCommerce website that supports PayPal. Buyers connect their PayPal account to the website before they get products. The website should collect funds without a normal checkout flow whenever buyers buy products.
For this, I've researched the Permissions Service API on PayPal. But, I can't find enough descriptions to build such a website. I would like to know how to integrate PayPal for this.
What you are looking for is a "Reference Transactions" solution. It can be tested in sandbox, but to use it in live, the business account would need to be approved for this feature by PayPal. To do this the account owner would need to contact their PayPal account manager or PayPal's general customer support (not MTS), and explain the business need for this feature.
As far as implementing the solution, the only public documentation I'm aware of is for classic APIs: https://developer.paypal.com/docs/classic/express-checkout/ec-set-up-reference-transactions/ . Any newer API or vault solution does not appear to be publicly available at this time, but you could always contact PayPal's support and ask if there's something they'd rather you implement than those classic APIs. The more significant hurdle is the business approval for the feature mentioned earlier.