Is a sales representative sinful for not seeking a ruling on selling goods he does not possess at the time of contracting, and is the money earned from this sale considered unlawful, and what must he do?

If the sale takes place after acquiring and possessing the goods, then these transactions are permissible, and the earnings are lawful. However, if the sale occurs before acquiring the goods, then this is impermissible.

If you undertook a forbidden sale, knowing it to be prohibited, then you are sinful, and a portion of your earnings, equivalent to these sales, becomes unlawful. You must repent and dispose of the unlawful portion by spending it on public welfare.

If you were ignorant of the religious ruling or the nature of the transaction, then you are sinful for failing to inquire. The ruling regarding your previous earnings in this situation is a matter of dispute, but the preponderant view is that they are lawful for you, based on the Almighty's saying: {So whoever receives an admonition from his Lord and desists, he may have what has already passed, and his affair is left to Allah.}