I bought heavy duty storage wracks at Sam's Club that can fit 5 longboxes per shelf and each unit has 4 shelves (plus a top shelf for light stuff - don't recommend loading that with longboxes, though).
I initially started ordering everything alphabetically, but I acquired many longboxes worth of comics after that so keeping things alphabetical no longer worked well. So I went to a numbered box format. When I am ready to inventory new acquisitions, I document them in ComicBase and assign a box number in the Grading Notes field. That way, when someone buys, the box number appears on the invoice, I can print the invoice and use it to find the book in the numbered box. I organize the books in each numbered box alphabetically so its easy for me to find each book in each box. I also have show stock (i.e. longboxes I take to shows) and I notate that show stock indicator in ComicBase as well in the Grading Notes field so if I sell one of those books online I can go to my show stock and find it. All of my show stock is alphabetized with the exception of some books broken out by cover artist or author.
I also assign a grade to every book I list on Atomic Avenue, so most of my books have an actual 10 point number grade (using Overstreet and CGC as my general guide). Those grades are stored on the Grading Notes of ComicBase and on a little sticker on each bagged/boarded comic. Assigning grades is totally a personal decision and I do it because I love grading comics and I want my customers to get what they are expecting.
Best of luck!