Steam Trading Cards that can no longer be bought/sold

Personally I would collect boosters and trade cards until I have full sets, then trade them for marketable sets with sets4sets bots and then sell those marketable sets instead. It doesn't matter to people leveling up if the sets can be sold or not.

Most of those games will have huge surplus of cards still going around since they are shovelware sold for fractions of a cent to huge card farmers, which is why they got banned from Steam market to begin with and they still drop boosters so you're not very likely to find anything valuable there. Of course there are exceptions to this where the game was removed for copyright etc reasons so those games are actually rare and cards might have some value.

I just turn them in to gems as it is far easier. If you want to trade them then you could always look up their previous 'value' on Steam Card Exchange (https://www.steamcardexchange.net/index.php) as a reference. You can't trade unmarketable cards with that bot but it will at least give you a starting point for what your cards might be worth.

Not much more to add than what has been said above except you can find some Steam Groups that are dedicated to unmarketable items. One such group is:
https://steamcommunity.com/groups/deaditems
I have never used the group (even though I'm a member) so can't say how good it is.

Use https://www.steamtradematcher.com
I traded alot here, many bots and users. The trade should be made manually, you choose what you need, 1:1 trades, even when a card is more expensive than the others from the set.

Thanks everyone for the comments. Appreciated ;-)