Hi there, I'm a new player,I need some info about redeeming Pack, in my inventory I have to choose but I checked,after the first time, if I don't choose when I open it ,I'm no more able to do later, is it correct?

I need someone who explains how theCommunity Marketplace i t works .
I know that I will use coins, earned with the selling cards, to buy my equipment , but
I need to engage a selling order with my card list first.

Please let me know what can I do with my player cards collection ?
How will I know the right price for selling each one ?

Thank you for help.