Having a boat full doesn't mean you can't place any more cards. It simply means that you can't place the new card without sacrificing other cards. That's the challenge of this aspect: you have to balance cards quantity and power. You will have to ask yourself: "Which card is the weakest in my deck (weakest for this archetype) that will allow me to place this card?"
But, I like the idea of being able to sell excess cards. This would reduce the impact of getting unlucky and would allow to get resources more easily (if cards are grinded into resources).