Selling unupgraded cards is almost always fine, if you have a purpose for the electrum, if you're just stockpiling electrum, you shouldn't be selling any cards.
Protip: read the tip of the year (that's probably been there for many years)
"Tip of the year:
Your deck can have a minimum of 30 cards; manage your deck and customize it as soon as possible, get rid of some of the cards you do not like/use and sell them. This way you can instantly have a better deck and some coins."