So it'd be nice to have a filter to automatically sell things that aren't usable by your class. For example a mage can't use "mail armor" but a mage can use a dagger if you get them trained. On the latter it might be cool if you were able to add a check for the training being present.

I believe Dejunk supports this.

You can easy make rules for that!

This Rule will Sell all "Item-Tokens" that are not for your Class:

(not IsEquipment) and (Level < 300) and IsSoulbound and (SubType == "Junk")  and TooltipContains("Classes:") and (not TooltipContains(PlayerClass()))

maybe need to be adjusted on non English clients and maybe also ItemLevel!

This simple Rule will sell equip that is not for your Class but keeps all Legendaries:

IsEquipment and IsSoulbound and (not IsEquippable) and (Quality < 5)

does TooltipContains work on content added to the tooltip by other addons?

does TooltipContains work on content added to the tooltip by other addons?

I don't think so, because there is not "the only way" to modify and also not "the only way" to read a ToolTip.
It may work for AddOn X but not for AddOn Y.