Perhaps this would work if, when you have a license, you can only collect salvage and deliver it to the appropriate authority. The premise in the Elite universe seems to be that all cargo is tagged and registered to somebody; if you don't have the 'paperwork' saying it is yours, you don't own it.
It would be unsurprising for the authorities to then say "you can't just pick stuff up and keep it", but also say "you can pick it up, but you have to give it to us, and we'll pay you a fraction of it's value". Typical government behavior - would be quite realistic, really
