Letters of marque worked when you could expect to get value from the permission it gave you. That matters wasn't just that you had a letter, but that whoever gave you the letter has the power to ensure you can use it. I could write you a letter of marque to steel cars, but the police will just ignore that letter (or arrest me for doing so - there are likely a few laws that could apply though I don't know them). If US Congress writes you a letter to steel a car, you can then take that stolen car and use/sell it in the US - the full power congress is behind you in saying you can do that (but don't drive the car to Canada or Mexico).
The important part here is I don't thing anyone can get enough value to be worth it. Often ships have negative value in a scrap yard - they are so full of toxic/hazardous things that scrap yards charge more than they are worth to cut them up.
Letters of marque worked when you could expect to get value from the permission it gave you. That matters wasn't just that you had a letter, but that whoever gave you the letter has the power to ensure you can use it. I could write you a letter of marque to steel cars, but the police will just ignore that letter (or arrest me for doing so - there are likely a few laws that could apply though I don't know them). If US Congress writes you a letter to steel a car, you can then take that stolen car and use/sell it in the US - the full power congress is behind you in saying you can do that (but don't drive the car to Canada or Mexico).
The important part here is I don't thing anyone can get enough value to be worth it. Often ships have negative value in a scrap yard - they are so full of toxic/hazardous things that scrap yards charge more than they are worth to cut them up.
Millions of barrels of oil or a shipment of coal is pretty valuable.