I figured it was just a problem with mission pass items, but now I have no clue why I can't sell this Aeros face for instance. Anyone know what's going on?

Frequency Bonus tickets from the AC Scratch usually cannot be traded since they are an incentive for people to draw from the Scratch themselves.

@AndrlCh Thank you. What about all of these cosmetics I can't sell?

Basically, if it doesn't come from the FUN Scratch, AC Scratch (not including Frequency Bonuses), or an enemy drop, odds are it cannot be sold.