Seems like a logical addon to exist, but I can't find anything...

Specifically an addon that let's you easily set a goal deck (or decks) and scans your inventory for what you already have, then creates a simple bark to put in Trade or other chat with what you are offering (cards in your inventory, including duplicates, that don't go towards goal decks) and what you want (missing cards for goals).

This could save quite a bit of time plus have the benefit of auto-updating after successful trades, unlike a traditional macro. I've seen other addons that auto-spam trade with a custom message, so it could even theoretically do that.

In general the only Darkmoon specific addon I came across was Darkmoon_Tracker, which is somewhat similar but requires other traders to be using the addon. It's cost tracking features are nice though, but could use some interface work. It seems odd that no ones's ever put together any sort of comprehensive card-tracker/trader addon.