One lady I know sells on Amazon and Half.com. She sells books that she buys at rummage sales. She won't pay more than $.50 for a book. She doesn't do all those romance novels, but she does do a few of the authors that are more well-known. She uses some kind of code and keeps Rubbermaid totes full of books that have the codes on them. Then she has the totes in her garage on shelving. She has a letter for each section of shelving and a number for each bin, so she might have 20-30 books with the code A-6. But that way she doesn't have to search through her whole garage. Alphabetizing by title or author is not foolproof - and if you add a book to your inventory, you have to possibly move 30 books over a bit to fit in a bigger book, moving some to another shelf, etc. This way, she can just add to wherever she has room.
