Just because you make money doesn't mean something public is bad. That add-on has been used and extended on by other developers, it is going to be better maintained and tested than something you will make.
If you insist on making something custom, you can check out Omnipay - https://github.com/thephpleague/omnipay. It's an SDK that has a wrapper around PayPal which you can use.
I used it in an early version of a control panel I was developing, but eventually ended up switching to directly using the PayPal API, for no specific reason other than wanting to directly interact with the payment gateway. PayPal's new API allows some more things to be possible and is, of course, a bit more flexible in the sense that you aren't following the SDK (Omnipay, in this case)'s patterns.
I don't want to use PayPal. I was simply asking if there was any documentation or an SDK out there somewhere for custom development of payment gateways for Xenforo.
I don't want to use PayPal. I was simply asking if there was any documentation or an SDK out there somewhere for custom development of payment gateways for Xenforo.