I had a look at my inventory earlier and found that after buying loads of packs to flip mostly Gold and Diamond cards, I've got an enormous number of Silver cards.
I'm hanging on to most of my Bronze and Common cards for exchanges and such, and of course because it takes forever to unload then for not much return.
Over the next whatever length of time it takes, I'm unloading all of my Silver cards at DEEP discounts. A quick scan through my teams shows that I've got from a few to dozens of something like 97% of all Silver card types, so it's going to take a few days to sell them all. At a rough guess, of the 206 Silver card types, I've got at least a few of 200 of them.
Anyway, if you're looking to fill some gaps with Silver cards, many are on the way.
Tip: extra Silver cards are perfectly suited to finish Team Affinity collections. I did all 30 teams entirely through exchanges.