First off, I don't know of this idea is common knowledge or not, so I might just have "come up" with an idea which I somehow never caught wind of from someone else, and now ignorantly think I'm a genius or something.
Anyways, I did some math and compared a few sites which can help you save money on ANY Steam game.
So let's say I want to buy an expensive steam game for about 200$.
Rather than pay 200$ outright, if I go to marketplace.tf, buy 200$ in keys, then sell those keys on the steam market, I'll generate a LOT more than 200$ I need for the game. This is because the marketplace.tf prices for keys are way lower than the steam market prices.
How lower? Only 36-37 cents. However, keep in mind I'm getting an expensive game, so I save 36 cents for EACH key if I buy keys from marketplace.tf, then resell them on the steam market (I'll need to sell about 80 keys on steam market at $2.49 to make $200).
So, how much money would I save on a 200$ game?
Over 35$ is saved in this case (assuming you can sell them all at $2.49). That's 17.5% saved (probably closer to 20% depending on my estimates and if key prices improve, which I'll talk about later).
Let's try a cheap game as well.
You can either pay 6$ upfront, or buy 2 keys for 4.24 on marketplace.tf and a tour of duty ticket for 0.99, sell them for 6.09 on steam market, and save $0.77 (plus 6 cents left in wallet). That's ~12% saved!
Now, something I want to point out before I list some pros and cons of this method is that the key price on marketplace.tf is super high right now. Normally, it's $2.05 or $2.10 at the MOST.
So, this would mean you could probably save closer to $40+ on a $200 game.
Pros:
Saves up to ~18%-20% on expensive games.
Saves up to ~%10- 12% on cheap games
Cons
Can take 1-2 hours to set up (you'll be on approving a bunch of market listings on your mobile authenticator)
Uses up to 80 - 100 steam market listings (you're only allowed to list 200 items on steam market each year)
No guarantee on exactly how much you'll save (key prices change often)
You'll have to wait 2-3 days to sell all your keys on the market.
Notes
This could also work with other sites were you can exchange money for in-game items such as opskins.com
If you have anything to add/point out mistake in my math, please do so! (y)