I highly, highly recommend using ITAD. It will save you a lot of $$$ while you will still be supporting the devs and publishers just as much as you would be if you bought from steam directly. For example, this plague tale game is $60AUD on Steam. While on an authorized reseller (Fanatical) for a steam copy, it's $51.55AUD. A difference of $8.5 might not seem like that much but take into fact that this game literally released today.
As time will go on, the steam price will stay the same and the prices on third party sites will drop immensely.