I've made a buyer account and a seller account. I've generated the button, pasted it into my code and tried it. Then proceed with the transaction and the page for success is showing. After that I log in with the seller account, but there isn't any activity.
On the page of the buyer the activity is present, but there is label '... hasn't accepted yet'. How to accept the payment from the seller account? Do I need to register a credit/debit card or bank account? What is missing?
I've did some research and without using your real paypal account and then log in developer.paypal.com you cannot test the functionality.
So it is a necessity to have a working PayPal account whose credentials you will use to log into developer.paypal.com and there you will have generated test accounts already.
In a few words: You cannot create a sandbox acc directly and use it.