What is the ruling on the money that PayPal returned to the buyer two years after completing the purchase of furniture, due to the seller's violation of regulatory terms, and must the buyer return the amount to the seller?

If the sale was completed and you received the item, then you have no right to the money that PayPal refunded. You must return it to the seller, because you have combined both the item and its price, and this is impermissible unless the seller voluntarily waives it, which has not happened. If it is established that the money belongs to the seller, it must be returned to him, even if he violated terms with the intermediary or the state; because this would be a financial penalty (ta'zir bi al-mal), which most scholars prohibit. Furthermore, it is money to which you have no right. If it is impossible to reach the seller, then you must give the money in charity on his behalf, and the reward will be for him, based on the statement of Sheikh al-Islam Ibn Taymiyyah that money whose owner is despaired of knowing its rightful owners should be given in charity on their behalf.