It’s good that we can learn any armor type on any character but please, add option “already known” to all armor types on any character because it’s hard to keep track.
There are two addons that can help you in the mean time
https://www.curseforge.com/wow/addons/trove-tally
https://www.curseforge.com/wow/addons/remix-ensemble-tracker
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Trove tally is really good for this
Haven’t checked but I’m guessing this is something they have in the works for the future, latest with warbands.
For people who don’t want to use addons (hi, I’m people) I suggest just making another character of the respective armor class and check what you still need. It’s also an extra step but if you’re going back and forth anyway…at least you don’t have to go to another server and manually search stuff.
What I personally would like to know more up front is which of them is NOT available in retail (up to that point in time ofc). I know not everything is in there, especially things you can still get are not sold, but it sucks I have to google/cross check things just so I don’t accidentally buy what I could farm outside remix anyway…unless I can get myself to fork over the loads of bronze required for what I could still farm 
I very much like that recolours and simpler things are cheap
They should have different vendors for each, similar to the one who jumps around in the 3 part class sets and weapons, simple and effective
you can use ctrl + leftclick on like transmogs and on the right side it will tell you that you own them already. its not perfect but a work around.