What is the ruling on not paying the remaining amount of the price of goods that I deliberately did not pay nearly five years ago, knowing that the seller's whereabouts are unknown and it is not known whether the amount was deducted from her or not?

You must hasten to repent to Allah for any shortcomings concerning the rights of others. One of the pillars of repentance is to return the financial right to its owner, or to give it as charity on their behalf if they cannot be identified. This right is not forfeited by the passage of time. If the seller was merely an employee, then the amount should be paid to those in charge of the shop if it is not their custom to deduct it from the employee.