What is the ruling on selling commercial spare parts in original packaging, knowing that the buyer will sell them as original? And is it permissible to sell a spare part at a higher price and write the part number of the more expensive item on the invoice?

Selling commercial parts packaged in original cartons, with the disclosure that they are commercial, is not sinful, unless you are the one who put them in those cartons, and provided that you do not know that the buyer will use them to deceive others, as that would be cooperation in sin and aggression. As for selling spare parts of lower quality while writing the numbers of a higher quality company on the invoice, this is forbidden deception, removes blessings, and necessitates writing the real company number, because honesty in selling is obligatory and a cause for blessings.