Comment by bluGill

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.