Many people wonder how highlighting affects their buyback value. 

Most major Textbook Wholesalers state that having more than 5 pages with highlighting can lower buyback value. Going beyond that amount can decrease value15-30% or in some cases, negate the value of the otherwise good book.

Your textbooks are an investment in your eduction and condition can be a large factor in it's resale value. 

Whether you highlight or not should be a decision based on your learning style as many people use highlighting as apart of their study process.  Know that if you are a prodigious highlighter, you may be affecting your resale value, but hopefully it is paying off with the knowledge you retain or the grades you receive!

When purchasing a used book, it pays to get the best copy of an item that you can.  If your school bookstore hands you a highlighted copy, ask for an unmarked or less-marked copy.  If you purchase online, make sure you read the description. Familiarize your self with the official definitions of conditions on each site you purchase on.  Amazon.com, eBay, Half.com, Biblio all list definitions in their "help" portions on their sites.  If the book you receive does not meet the minimum definitions, you have the right to receive a refund or return.   

Water/ liquid damage happens to a book when it is exposed to a liquid and then dries.  Various liquids can stain the pages and cause the pages to become wavy, stuck together, or even cause mold.  The best practice to prevent a book from becoming damaged is to quickly blot a spill, if the liquid has started to get absorbed, place paper towels between the affected pages and air-dry.  To prevent waviness, place something like a board on top of the damaged area when the pages are partially dry.