It was existing to get my first rear card, but after some time i just sold my rears to upgrade my cards. I think u shouldn't keep the rear cards unles u gana need them.
Actually it`s better to keep rare cards even if you don`t use them at the moment. At some moment you are going to need them and farming might be really hard.
Another thing is that amount of electrum you get from selling one rare is ~250 which is equal to 20-25 wins versus AI3 or 3 wins in PvP. I don`t know if there are statistics how often rare cards are dropped but i doubt if it is profitable to sell one.
Well... In fact you shouldn`t sell any crds unless you have to many of them. I recommend
this tutorial
