When I first started selling online, I just listed my books, put them into clean liquor boxes, and waited for sales. When the sales rolled in, I then had a scavenger hunt to find the book. By the time my stock had grown to about 20 boxes, things were getting out of hand, and I knew I had to find a better way to organize my inventory.
Thankfully, I found it!
Now when I write a listing, I make a 3" x 5" card for each item. On the card I write:
Name of item
Digital photo names (numbers)
File where photos are located
Box # where item is
I keep these cards filed in colorful plastic shoeboxes that I bought inexpensively at Target. They are filed alphabetically by item name.
All of my books are in numbered boxes. If I sell the book Betty Crocker's Cooking Calendar, I can pull the index card, see it is in box #31, pull the book, and voila, I'm ready to pack!
It is also easy to delete photos, as you know exactly where they are on your computer.
I've used this method for years and I swear by it as a simple way to organize books that have been listed online.