Selling ships I would say no to. If I was able to, I would sell ships on a far away planet to put more industry closer to my borders. The game would become unbalanced when people start doing this. Instead of sending your ships across the map, all you would do is sell the ships to buy more industry closer to your borders.
Selling carriers I would be OK with if it is 50% of the price. If you don’t have very much money but need to defend your borders, the carrier way on the other side of the map won’t be much use. You can sell 2 carriers that aren’t needed right away to defend your borders or attack if you need to. This will also prevent players being unable to defend when it is near the end of their turn and they already spent their money with an attack coming. I never seem to have extra money. Selling used carriers for money seems like a logical explanation for the money. The price diminishes after you buy the carrier for the full price so when you sell the carrier, you only get half the money back.