Very simple question about TF2 keys

Because the price is different in different countrys. Games in € countrys are most times the most expensive, $ is a bit cheaper and russia and brasil are super cheap

Im not 100% sure either. But I think its because of Russian sellers, they got diffrent pricess from us. And its much easier for them sell those keys on market.

ahh right, so why not use a direct russia seller instead of keys?

Different countries have different game pricing. So as TF2 keys is the global steam trading currency, it is widely accepted by everyone. You buy keys and not send the money directly because there are chances of getting scammed. If you have keys, you can just trade it for the game.

The reason they chose keys as payment is also that they are pretty constant in selling/buying price. They don't vary as much as other objects do.

Yep thats true. Sometimes do this.

Games don't become cheaper, you just buy them at prvious sale price or Russian price if not region locked.

I read a lot of things. What I can tell u is that as anything on any market, as soon as it is bought it price go cheaper. U will never buy a game from any 1 at full price, therefore if they have a game and want to sell it, they will ask for lower price.
If u think "then wth do they get from buying the game and reselling for keys?" thats very easy. The last bump on these trades was CS:GO. It were at 2€ for a full weekend, 2€ buys 1 key and sweets, now they sell it for 3 keys = 5.5€ or something.. So they do gained 3.5€ from doing it. Then as u see they buy games during sales and sell during its full price so they actually gain. U have 2 options then, to buy it for the 12€ of market or for the 5.5€ = 3keys from some 1.

As people have said, you trade the keys for games that were usually bought in other regions or on sale (or both).

Doing so lets you trade at their price, a $10 might be going for 3 Keys on the trade boards; and since Keys are $2.00-2.50 you can save 25-40% by doing this.

As for "why not send cash"? The Steam trade window is much safer as Steam will give you back your keys if you traded with a scammer. It's also much easier and safer as no one has to "go first".

A Simple hands on example.
I just traded 7 keys and 1 ref for Transistor + OST preorder with a Russian trader over at Steamtrades. It only cost about 1/3 of the Euro price.