Soldato
You need the steam app and 2FA on you phone AFAIK, when you go to sell on the marketplace it then pops up on your phone to confirm it.
Again AFAIK it takes 7 days to authorise that too, after then you can just flog whatever you want rather than paying to open crates. Better to get the £2 into your steam account than having useless cosmetic items tbh
I have the app on my phone, logged in, but when I try and list a crate it says it wont be listed for 15 days as I've not used the authentication for 7 days. Well ask me to use to use it then surely?! When you click learn more, it basically says install the app. I have!?