You’ve got two main options: use the MassPay API or Adaptive Payments API.
Note: Adaptive Payments is not available for new integrations.
PayPal provides documentation to only support existing integrations
The Adaptive Payments option gives you a lot more control over the payment and is the future direction of the Paypal APIs. It supports SOAP, NVP, JSON and plain XML. MassPay has both a SOAP and NVP interface.
IPNs are a totally different subject: https://www.paypal.com/ipn
Seems like lot of things are now deprecated. You should use PayPal Payouts.
Use PayPal Payouts to send money to multiple people at the same time.
To send a payout, you need to know:
- Each recipient’s email address, a domestic mobile number, or payer ID (an encrypted PayPal account number)
- Payment amount (per recipient)
- Currency code (one currency per payout)