Boarding a boat bigger the the one we have should be hard with the difference of crew number no ?
For the money the objective for having a lot in pirate! was to stash it in you secret cave (I really like seeing the amount of gold getting bigger and bigger) for you retirement, so maybe adding a way to "invest" it like buying land and why not build stuff on it (plantation, manor, slave/servant) and the costly maintenance. Of course pirate can't be officially owning land so we have to hide/stash somewhere for not looking kilos or tons of gold if we lost or ship (unless the gold in game is stored in a magical place :p)
As for selling boarded ship it should be possible only on some pirate shipyard because sincevyoi didn't have it legally you don't have a paper that prove you the owner, at the difference of buying one.
Maybe add some upgrade that you can buy for the boat like in the recent sid meiers pirate .
Buying the rank when we evolve for a nation. The higher rank the more we need.