Every book I sell is a used book, and has either been thrifted, is being sold on commission, or is simply a book that I no longer have space for on my shelves. Usually, these books are in very good used condition - sometimes they look almost new. Sometimes they show considerably more wear, though they are still good for many more reads. Here's a list of the abbreviations you'll see on this site.
SC - soft cover/paperback
HC - hardcover
BB - board book
LB - book has an extra reinforced library binding
dj - book comes with a dust jacket*
*Dust jacket condition is always described separately. Sometimes a dust jacket is in poor condition because it's been doing a great job of protecting the book underneath.*
EXC or like new - book is in excellent condition - like new - with basically no flaws
VG - book is in very good condition; it may have minor flaws such as minor creasing to the covers or a few dog eared pages, it might be inscribed, or have dents or shelf wear to the covers and spine, but the binding is sound, and book has no major defects
G - Good condition - evidence of wear and usage (scuffs, staining, creases, bumped corners, markings, minor tears, etc.) but a solid reading copy. Any flaws do not interfere with reading. Good condition books might benefit by having a piece of tape placed over softcover spines, or the hinges reinforced for longevity. etc.
F - Fair condition books are readable but are not books to hand down to your descendants. There could be more substantial damage, such as the spine separating form the cover, tears through multiple pages, lots of underlining or highlighting, etc., in addition to the potential issues mentioned above.
P - Poor: book has many flaws and is better for crafting than reading
exlib - ex-library book - not a condition descriptor as such. An exlib book will have library name/discard stamps, stickers, pockets, etc.
vintage condition - not necessarily a descriptor of condition, but more an understanding that glue may be brittle, the boards may show wear, there may be some foxing, etc. A book in good vintage condition is a book that is in good condition, considering it's age and the amount of usage it's seen.