Has anyone used the "Auto mark unused for trade" from the Inventory screen and, if so, how were your results?
Accurate? Nothing messed up in inventory/tradeable status/decks?

I see the warning prompt clarifying what will happen, which is what I want to ultimately do...just wondering if the outcome was a good one for those that used it before I pull the trigger on this. :)

I have! It all works out. Take a backup if you feel nervous!

I have some cards that I don't want to trade even if they are not in a deck. I have a "Do not trade" tag on these, and what I do is turn the Auto-Mark for Trade feature on, and then I turn it back off again and go filter the trade list by that tag and remove them from the list.