Short answer: Treat any condition label as a starting point, then check the photos and ask the seller about anything the label does not cover: missing pages, writing inside and loose binding.
Condition labels are not a standard
Words such as "good", "fair" or "like new" are the seller's own judgement. One seller may call a lightly highlighted book "good" while another calls the same copy "fair". Read the label as a hint and let the photos and the written details decide.
Look for specifics. A description that names what is wrong, such as "pencil notes on 20 pages" or "cover corner bent", tells you more than a single grade word. A listing with no detail at all is a reason to ask a question before you order.
What to check in the photos
Open every photo and look at the cover, spine, edges and a few inside pages. A spine that is cracked or a cover that is peeling will not improve with use, especially for a thick textbook you will open daily.
Check whether inside pages show the writing you can live with. Light pencil underlining is usually easy to ignore. Heavy pen marking, answers filled into practice questions or torn corners make a book harder to use, particularly a problem book.
Things photos cannot show
Missing pages, repeated pages and loose binding are hard to see in pictures. Ask the seller directly whether all pages are present and whether the binding is firm. Also confirm the edition and year, because a book in perfect condition is still the wrong purchase if the syllabus has moved on.
Smell and dampness are another gap. If a listing mentions water marks or a damp storage history, decide whether that matters to you. For long-term study books, many students prefer to skip those copies.
Common mistakes buyers make
The most common mistake is ordering on price alone. A very cheap copy with heavy damage can cost you more in the end if you have to buy another one. Another mistake is ignoring the edition because the cover looks right.
Buyers also forget to compare delivery. Since the buyer pays delivery on top of the book price, a slightly higher priced book in better condition can be the better deal overall. Take a minute to add the totals before you decide.
Match condition to how you will use the book
A reference book you will keep on a shelf can tolerate a worn cover. A workbook you plan to write in is fine with older pencil marks. A mock-test book you want to attempt fresh is better bought new, because earlier answers spoil the practice.
It also helps to compare the price gap with the new price. If the used copy saves only a small amount and has visible damage, a new copy may be the better choice. You can compare both on second hand books and new books.
Frequently asked questions
Who decides the condition of a used book on a listing?
The seller describes it, so wording differs from seller to seller. Use the photos and ask questions when a detail is unclear.
Is a book with light highlighting acceptable?
For most concept books it is fine. For problem books and mock tests, pre-marked answers can reduce their value, so check how much is filled in.
What should I ask before ordering a used book?
Ask whether all pages are present, whether the binding is firm, which edition and year it is, and whether there is any water damage or heavy writing.