where to sell steam retail keys without providing business documents

No, all those websites are legit businesses and follow the laws which require them to 'know their customer'. Your second best bet is using sites like steamtrades.com and sell keys with paypal(or similar) transfers.

"also is it good idea to provide them with fake docs"
Genuine question, are you stupid? Just read what you wrote back to yourself.

are you selling your own game? or just a handful of old bundle leftovers -not worth it for the second one

is it possible to be a seller at kinguin or g2a or any reselling website without providing Certificate of incorporation or Business registration docs

No. It's their policy to only allow Businesses to sell there.

I don't sell in that category, but eBay has a "Steam Games" category.
I'm US based, and I sell old tools, electronics, and other things on eBAY for thousands a year. I didn't have to incorporate for that. I just have to pay taxes.

also is it good idea to provide them with fake docs ,know that i want to sell legit keys

That's fraud. It's never a good idea to break the law.

the only issue is that i cant get those documents and they are very hard to get in my country

Perhaps you could come to a arrangement with someone who has the correct papers? A family, a friend, ect? You could act as their agent and maybe pass a percentage to them.

Gameflip will let you. But you have to dox yourself. Picture of ID, face, ect.

I have never done business with them.