Inside hack

Dear Steam / Valve.

I would like to find out how it is possible for someone to gain access to my account? I have two factor authentication setup (did not get any prompts of new logins) Then for whom ever to sell everything sellable from my inventory with out me having to approve the sale on the app? It was 159 transactions. (this is not normal behaviour). I would like for you to explain this to me please. And then while you at it, I would like my items returned to me.