I`m not sure if I understand your questions right.
I sell cards on the market if I have them more than 2 times normally. I always keep 2 for myself. But it depends which card it is, some cards you need for your game, others you don´t.

The longer and more you play and the more cards you have, the higher is the income of starbits you get for the missions. So it takes quite a while in the beginning, but it´s worth it!