Never buy books until after the first class, except maybe math textbooks.

Also your mileage will vary depending on the book/edition, but a lot of times a "new edition" of a textbook is just a transparent cash grab by the publisher and is 99% the same material with different page numbers, so it's worth asking the prof/a TA if the previous edition is pretty much the same. You can generally get "outdated" editions of a textbook for startlingly little money. Like I'm talking sub-$5 for a book that's $140 new sometimes.

When I was a TA for a gigantic intro class they'd just released a new edition of the book we used but they'd only sent us two desk copies (publishers send free copies to professors who teach out of their textbooks), and the class was run by a professor and three TAs, so the TAs all had to share one copy of the new edition and taught out of the old edition 90% of the time. They'd only changed one chapter, so the professor scanned that one chapter to PDF and we handed it out to anyone with the old edition.

We also had, for some reason, like five boxes of the old edition under a desk in the department office and gave them out to anyone who would take them. You can hardly give old editions of textbooks away.