Here's a suggestion I'd find pretty useful, myself:
Have a column in the inventory view that has an icon to indicate if that card is in a deck from your deck list. When the icon is moused-over, have a little popup show which deck(s) that card is in and how many.
ex:
<- icon to mouseover
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Found in the following decks:
Sigarda EDH [1]
Green/White assault [4]
Green Trample [2]
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This would be much faster than the current method, which is to click on the card itself in the inventory, where it then takes you to a new page that shows what decks you have with that card in it. Anyway the site is awesome and keep up the good work! I use it every day.