i used to sell my school books on ebay, what i did was i first checked how much the school was willing to pay me, which is crap. so i checked how much i paid for it used. i tend to save my receipts because of financial aid reasons, so i had the receipts, then if i want something back besides the crap school is offering, i'd list it at half of what i paid for it... because people check online for cheap prices, not retail for used books. and key thing, when i buy my books i search for grade A used books so i can sell them at grade A used for good money, sometimes i get 100% + some back because of that. if it's a new book, i'll list it new if i didn't write in the book haha... sneeky but works. i've sold probably about 10 books over the years.
i usually post a flat $5 shipping fee, books are considered media, and media shipping is cheap, so it's no biggy!
i don't remember the most i've gotten for books on ebay... it's been a while and i have bad memory, but it was about 60-70 for a book. better than school's $10 offer for sure.