Steam wallet balance has been stolen

do you entered case site with steam account

If you dont have steam guard up, you are on your own.

Only thing you can do, is learn from it, and don't make the same mistake again, which is logging into third party scam sites

and before you say no no, they're all legit, there's a reason Valve censor their names on Steam

Your money is gone

this is why i dont keep a balance and always delete or never save payment info.

If you enter your steam login data on another site, steam guard is useless. You've gifted your money to the scammers.

There are two ways to prevent this:

1. (weak) Always login to steam independently using the real steam start page and only then login to the site, not entering any data.
2. (strong) Don't go to scam websites at all. If people stopped going there, the scammers would stop creating and advertising them, and the world would be perfect.

So steam basically told you this.

Sorry you got scammed.

Kinda weird Steam cant undo the trade, but how are they to know you're not scamming someone and abusing the situation. Easier for them to just nope right out, and not possibly make the situation worse.

How much was lost?

They probably hit you with the API key scam
go here and revoke it: https://steamcommunity.com/dev/apikey

Or they stole your browser cookies

That is if you didn't logged in a fake steam site

Change your password now. You will be logged out from all devices that way. From now on, make sure to only login to steam whenever you are sure that it is not a scam site. If you are already logged in on steam but the website says that you are not, get out of there immediately. Or you can always type the steam website manually and login there before logging in through third party websites or similar looking ones. Some url like store.stearnpowered.com and store.steampowered.com can look similar at first glance, albeit different.

Iam sorry this happened to you but it is your own fault. Do not share your login credentials with anyone, trying to log in on fake site is sharing. If you get any message related to CSGO then you can be sure that is probably scamming attempt.

Only thing you can do is to go to the Police, as this is a cybercrime(no clue how "huge" was that wallet, but if some serious amount the might be worth to do that, just for statistical purpose.). Yeah, nothing will happen, but eventually they might force valve to track the way of the money. However, if everyone would contact police who got scammed by these CSGO scammers then sooner or later it would have effect: some law regulations and forcing valve to implement some changes (like the one i mentiones at next paragraph) because of the amount of police cases.

Valve also could solve this. NO item should be purchasable by selecting the price and seller, not even skins, you should just purchase the cheapest, and if something is 0,03€ and you set up 50€ purchase value, still the cheapest should be purchased and only 0,03€ should be deducted.

Choose any 3-cent item.
Offer 50 cents.
See what happens.
Reread what you wrote.

Edit, from a reply:
"Then he can only go to the police and then send a letter (not an email) of formal notice to Steam's legal department, to bypass normal support."
Nothing is lost, but it will take time.

The comments calling you a hacker on your profile are pretty funny too.

i feel ya, i got hacked 2 years ago, they stole like 3k inventory items from me. steam basicly told me "shit happens"

You entered your credentials on some phishing site (pretending to be steam). Since it was your fault - steam won't help you to return balance or items. You can try to go to police and ask them to find scumbag who scammed you, but IMHO that has very low chance of success, since you don't even remember which site it was, and police rarely work well with cyber-crimes.
So, all you can do is change your password, check if you have api key on https://steamcommunity.com/dev/apikey and revoke it if you don't remember creating it, and optionally change password on your mail connected to steam too. And be more careful in the future.