When I was in the same situation, but I also could not understand. The only thing that occurred to me was to save important information for further transactions, and then set up a cronjob using a script that transfers money from your account to the party.
Example: CustomerA wants to buy SiteOperator from you through CustomerB
CustomerA offers a free article from SiteOperator
You store information about the amount that you need to send to CustomerB (calculate the fee, etc.)
Cronjob cycle every 15 minutes and calls the PayPal API to send money to CustomerB.
This is how I do it.
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