Legal Question in Real Estate Law in California

Yes, you can sell the car part, but you must do so in a "commercially-reasonable manner" and the net proceeds, after deducting your costs, belong to the guy that left the car part in your car. To be perfectly legal, you should mail him a check for the net proceeds, along with an explanation. A commercially-reasonable sale may be almost anything, depending upon the marketplace, that is likely to get a fair price, and might include an eBay auction, selling it to a used parts dealer, back to the manufacturer, or whatever is fair to the owner (and yourselves). You can't just give it away (unless it is junk).