First, don't buy from the bookstore. Buy online.
Second, don't assume you have to have the latest version. In other words, go online and buy the 4th edition instead of the 5th edition; the 10th edition instead of the 9th edition, etc. In most fields (some technology ones excluded), the updates are marginal. I've found, for example, that a 4th edition might sell for $8.50 when the 5th edition is selling for $85.
You can find out from the publishing company what the changes in the text are. You can find this out at the publisher's Web site or you can call the publisher and ask. You would laugh (well, you guys prob'ly wouldn't) if you saw what the changes are. Most of the time they don't amount to a hill of beans and they certainly aren't worth the difference in the price, which can be as much as 40-60.
Tell your instructor what you're doing and why. Depending on how knowledgeable the instructor is about this type of thing, he/she will probably be understanding, especially if you cite the poverty status of students as the reason.
Make a photocopy of the first chapter from a colleague's copy so you don't get behind on the reading while you're waiting for the book to show up from your online source. Tell your instructor that's what you're doing.
When you go online, put the title of the book and the author right in the Google search box. It will turn up a lot of entries that let you comparison shop the copies.
Check Amazon and eBay, too.
Also, those special cellophane package deals are usually a rip. They have databases that the student is already getting through the library. It's just another huckster deal to scam you out of your money.
Sometimes departments have a custom publishing job and they tell you that you have to buy the special edition of the book. Find out what the difference is in the custom job vs. the regular edition. Usually it's some lame thing like an appendix. Whoopee. You can photocopy that from someone who bought the custom edition.
Also, don't begrudge your professor for using his/her own books. Believe me, profs don't make that much on the publication of a book and you actually are lucky to have someone as a teacher who has also published. He or she is probably one of the experts in the field.
When it comes to re-selling your books, don't take them back to the bookstore. Sell them online. Start listing them a month before the semester starts. There's a bit involved in selling books at a place like Amazon, so get familiar with the process way before you actually have to do it.
Cher