some of you already noticed the "new" update Steam did on the VAC-System including the Steam Guard.
But for those who doesn't here is a short summary.
"If an account that has a phone number registered to it receives a VAC or Game ban, all other accounts that used the same phone number at the time of the infraction will also receive a ban"
So be careful which account you linked with the steam guard.
E.g. you play CSGO and you got one main account with high tier skins and one account without skins for cheating and you got ow'ed with that account AND both accounts are linked to the same phone number, your main account with the skins get ow'ed too and you wont be able to trade or sell them.
Hope it was helpful for you,
Cheat on !