They majorly purchase through PayPal but also buy large amounts through other payment options like WU. They then sell through PA and other demanding payment options. They never sell directly through PayPal without asking for identity card, etc I believe.
I don't think you can just jump into it, you have be trusted first of all so that people know that you won't charge back your payment through PayPal and if you were to buy from WU constantly, you wouldn't be making much or barely any profit. You can always give it a try though. Start small, build a customer base and grow. All the big merchants you see in the GP market all started where you are at.