I just use an Excel spreadsheet to keep track of my inventory. I use different tabs for each of the major sets that I collect, and then generic tabs like "W Cards" for random cards that I have. I also have different tabs for when I sell cards, and how much they sold for etc, and also to track submissions to TPG's. Here's a tab of my inventory for the 1933 Goudey set, which I used to collect, but I've been selling off.
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