Allow the current “Import .csv” function (at least for Moxfield .CSV format) to accept negative numbers in “count”. And for those cases remove the card from your collection instead of adding. Would help a lot for when you’re selling or trading large amount of cards to not have to remember to go and manually delete them noe by one!
I think this is a great idea, but adding a warning before deleting would be wise, a mistyped minus could otherwise wipe out cards, which would be easy to not notice and frustrating to fix.