Is there even a market for non marketable trading cards?

There's a group for this: https://steamcommunity.com/groups/deaditems
Also, posting in the game's trading forum gets you offers regularly. That's what I do mostly.

https://www.steamcardexchange.net/
https://steamcommunity.com/groups/card-trading-card-trades

Trade them for gems

https://steamcommunity.com/groups/tradingcards/discussions

There are a couple traders on SteamTrades that'll buy them, but you're only gonna get a handful of gems for each. Better to trade them to badge hunters when you can find them.

https://www.steamgifts.com/discussion/qfEUj/rudobot-cross-set-trade-of-non-marketable-cards

https://www.steamgifts.com/discussion/qfEUj/rudobot-cross-set-trade-of-non-marketable-cards

Yeah and they can be much valuable. I assume they are more and more valuable the longer from revocation date.

First place is if in the game's discussion page (community) they have a trading channel, post there your offer; if they don't post in general.
Then there are several groups for trading non-marketable items. I have at least two, if I find the third (I think I have three) I'll try to remember to post it here.
https://steamcommunity.com/groups/deaditems
https://steamcommunity.com/groups/Non-Marketable

There are also trading bots accepting non-marketable cards but I lost all my links.

In any place I guess you want to trade non-marketable for marketable. My suggestion is to specifically ask for marketable cards, unless you're interested in other kind of items.

Good luck!

BIG thanks to every1! yall been super helpful! Sorry for the late thanking!