If a game on Steam is removed from the store, then all of its items and cards become unmarketable. It means you cannot sell them on Steam market, but you can still trade them with other Steam users. So how to sell these items and cards?
I looked for a solution and there are a few.
Turn into Gems…
You can turn your not marketable Steam items into gems. You will get around 4-20 gems per trading card, but it’s absurdly not worth it. You need 1000 gems to turn them into a Sack of Gems, which is only worth around $0.4.
Exchange trading cards
You can exchange these trading cards or other items with your Steam friends. If you don’t have friends then you can join a Steam Trading Cards Group and search or post a trade on the forums.
Sell your items
This is the way. You want to get Steam Wallet money for these unmarketable cards and items. Some of these cards might be worth a little bit as there are Steam badge collectors and they’re looking to complete their badges by getting five sets of trading cards from these removed games. You can target them by looking at steam groups or reddit.
The easiest way I found is service like SteamLvlUp, which primarily helps with leveling up your Steam profile by completing trading cards sets etc, but you can use it to deposit your not marketable items and cards using their bot and sell them for gems. Personally I had about 70 unmarketable trading cards and I sold them for about 6000 gems, which then I used to purchase 20 sets of trading cards for CS:GO. Each of them is worth around $0.20 and I sold these cards via Steam Market. I’m pretty sure it’s not the best value you can get, but it’s super easy and fast to do.