If an item is salable at all, I try to use the following divisions:
Please note: Hardcovers have 2 grades. The first grade applies to the book, and the 2nd applies to the dustjacket.
When I say "1st" for printing - that is assumed to be for the particular format of the book for sale, as in "first-paperback printing" (it may follow a hardcover from an earlier year.) For SFBC editions I usually leave printing number and price blank, unless I know that it is the first hardcover edition (then I note "1st".) I try not to identify printings by later publishers in a given format as "1st." [The difference between "first edition" and "first printing" can get involved - I assume most people know the range of possibilities.]
When pricing, I try to factor in desirability, scarcity, and condition. Price generally follows condition: below $2, condition tends to fall off (look at the gradings listed in a price-range to see what I mean.)
Anything on this list may be put up for auction on eBay at any time, and the auction will take precedence. (Note that listings that have detailed descriptions and plot/contents would be particularly easy to turn into auctions.)
With this many books, I don't have the time to create plots/contents or detailed condition descriptions for everything. About 1/4 to 1/3 have plots/contents-lists, and maybe 10% have detailed condition descriptions. Those tend to be titles I've sold on eBay before, or listed on eBay but didn't sell, or collections (I am an expert on "single-author" SF collections.) Lately I've found that a decent scan of the front and back, along with my grading - is adequate for most books. If I haven't read the book, the back cover may tell you as much as I would know about it.
If you've seen my eBay auctions, the detailed descriptions include what I consider to be the supporting evidence for my grading. Below is a long-winded version of how I arrive at these grades.
Grading is self-taught: I picked up these terms from buying from other booksellers - often online. I model my terms on those used by sellers who seemed to be consistent and accurate. "Good" for some sellers included such enconiums as "cover attached." (So there may be a fair amount of noticeable wear on a book graded "Good.") I prefer not to sell books in Poor or Fair condition, and would rather sell books that grade VG+ or better - but some rare/desirable items will have more wear and still be worth selling. A book graded "Good" or "VG" would make a reasonable "reading" copy - but probably fall below the standards of a really serious collector.
Basically, I pick up a book and a grade pops into mind (my reaction.) Then I look closer and check for details to support my assessment, and adjust if needed. I will often blur the grading/category if condition seems to fall in the middle (like: VG+ to near-Fine, or Fine-, or Good+)
I consider a "Fine" copy to be something like a book that you bought new and put on the bookshelf unread. It may have a little bit of wear, but nothing major enough to knock it off that grade. Some wear is allowable, or it would be graded "Mint." For a grading of "Mint" I imagine the equivalent of picking through 10 copies at the bookstore to find the best - it must be in obvious excellent condition (very tight and square and/or sharp edges, etc.)
For a book to grade VG+, it has to be a pretty nice copy, with some obvious but not terrible flaw(s) to knock it off a "Fine" grading: like a mild tilt, or line down middle of spine, or light overall wear.
As flaws accumulate, I work my way down the list, trying to keep in mind the severity, type, and number of flaws.
The things I fear most in terms of condition (and wish to avoid when buying for myself) - are water damage or stains, underlining, or excessive browning of pages (to the point of crumbly page edges.) I try to avoid selling such items, but sometimes stain-traces may be minor or limited-enough to be tolerated if the book is desirable. (Although I will usually knock down the grade a notch or 2, and describe as I would want it described to me if I were considering buying.) That'a my other rule, or underlying policy: "no surprises."