Sell silver enhancment cards?

Well, they yield only 8k XP points, so they're pretty bad. Usually is better to burn them for mana prism than leveling servants with them.

To add to the previous response on top of Mana prisms > 8k xp it's also a waste of qp which you'll be needing a lot of.

They are less QP efficient than the gold exp cards. At some point (perhaps around level 40 or so), using one silver exp card costs more QP than it nets you exp for the servant.

If you have more than enough QP (and don't plan to level any 5* servant anytime soon), and you don't need any mana prisms either, you can still use them.

Personally, I usually throw even all my bronze trash at my low level servants and save the gold cards for leveling the high tier ones. I sell the silver cards because I'm saving for 5000 mana prisms to purchase the upcoming CE in the shop.

Just to expand on what has already been said, every time a servant gains a level, it gets more of expensive (in terms of qp cost) to feed him more exp cards/fous. So if the servants are higher level, it costs more QP to enhance them further, so if you want to be efficient with your QP, you should feed them as much exp in one enhancement as possible (thus, 20 gold cards at a time).

It you don't care about the QP efficiency, and you need exp more than you need mana prisms, then go ahead and use the silver cards. It's not like using them will have detrimental effects on your servants or anything. Personally, I used the silver cards that I got from the event shop on Chloe, because I didn't have a surplus of extra cards lying around, and we need to acend her 3 times for the event (plus she is useful for the event). However, to try to maximize QP efficiency, I fed her the silver cards first, starting with when she was level 1. Then I started feeding her the gold cards once I ran out of silver.