I think it would be useful to filter the cards in my inventory by whether they are:
- used/not used in (built) decks
- not used in tradelist

If this is combined into a single "used/unused" filter, that'd be fine too. Either feature set would allow users to manually mark unused cards for trade, for those for whom the "Auto mark unused for trade" feature is too aggressive.

Would anyone else benefit from this?

Yes, I also think the following features would be useful:
- Filter based on presence/count in built decks
- Filter based on presence/count in tradelist
- NEW: Export count in built decks as an extra (optional) column in CSV
- (The count in tradelist column is already present)

I've actually submitted a "Support & Feedback" about this before seeing your post. That's good to know I'm not alone.

"- NEW: Export count in built decks as an extra (optional) column in CSV"

Just what I was looking for. +1 this request!