If you are active in the games’ auxiliary activities such as the fiction forum and the occasional fiction contests, the art events, and the occasional other contests and events, you have probably at some point received cards as a reward or prize.
The question under consideration is whether you should keep said cards and use them in your arsenal, or should you sell them for their Morphon return value?
As with so many considerations that this game leads one to deal with, the answer is “It Depends...”
If you haven’t reached a point in your leveling/card activating by team-mates where all activation slots are full, it would probably be to your advantage to retain them for use as powers.
On the other hand, if your actual card count exceeds your activated cards by a fair margin, you might consider selling some of the excess ones for their Morphon return value. This is my own current situation.
Look also at the values of the cards. Does the Attack, Defense, and Movement gains you get from owning the card outweigh the value of the card you want to purchase. Most reward cards, like Reality Writer are very low value for their bonuses, but some cards, like Stop the Presses are fairly good cards numerically. I would also like to point out that reward cards are not meant to be leveled.
Whichever decision you make, there will be pros and cons to deal with, so do not act without careful consideration.