Sell the week your exam ends
Buyback vendors buy on a cycle. They restock heavily at the end of each term, when they are forecasting demand for the semester about to start and their shelves are empty. That is when quotes are highest.
Wait a few weeks and you are selling into a market that has already filled its orders. The same book, in the same condition, with the same ISBN, can be worth noticeably less in July than it was in May — not because anything changed about your copy, but because the buyer no longer needs it.
The new edition problem
Publishers reissue major textbooks every two to four years. When a new edition ships, courses switch to it, and demand for the previous edition falls off a cliff. A book worth $45 in May can be worth close to nothing the following spring if the course has moved on.
This is the single most expensive mistake in textbook selling: keeping a book you are finished with because you might want it later, and discovering a year on that it has no market. If you are genuinely unsure whether you will need it again, price out what keeping it costs you. Often the answer is most of its value.
The rough value curve
End of term, immediately: peak. Vendors are restocking and competing.
Mid-summer: noticeably softer. Quotas are filling and urgency has gone.
Just before the next semester: a partial recovery as vendors top up, though usually not back to the end-of-term peak.
After a new edition is announced: sharp drop, and it does not come back.
What this means in practice
Check offers during finals week, before you have even packed. Sell anything you are confident you will not reference again. Keep only books in your own field that you have a specific reason to keep — and understand that choice is costing you their current resale value, which is a fair price to pay for a book you will actually open again.
One exception worth knowing
If your book is required for a course you are taking next semester anyway, obviously keep it. And if you are in a sequence — organic chemistry I and II often share a text — check whether the same book carries over before selling at the end of the first term, since buying it back in January will cost more than you received.