When the seller is paid, the customer's payment is credited or debited?

Credited.

When a customer purchase any product or service, he pays to the seller of that product or service. He is giving money to the seller in exchange of that product or service. So while making financial statements, whenever money flows out of the account, the account is credited with that amount. Similarly whenever money flows in to the account, the account is debited with that amount.

However in real bank transactions, it is done exactly opposite of what is being read in financial books. When money flows in, account is credited and when money flows out, account is debited.

depends on what type of card you use.

is it a debit or credit card?