Misunderstanding #1: I've mentioned this before on my blog, but our students - even at the elite colleges and universities - are not us at that age. They are at college mostly because it's the next logical step and they've been told it will help them get a better job and so on. Even the most academically-oriented will usually only buy books for classes in their major, when they know they'll want to keep them beyond the class.
Misunderstanding #2: Two of my three library jobs have been at colleges with a high proportion of first generation college students. In both cases, more than 40% of the undergraduate population were/are first gen. They don't come to college knowing the culture, the expectations, the norms. They were given access to their textbooks in classes in k-12, and some even got to take them home, and so they won't expect this cost.
Misunderstanding #3: Speaking of costs, have you seen how expensive textbooks are these days? The prices have risen way way way beyond normal inflation. I've seen 150 page, flimsy paperback textbooks going for $100+. You know how so many libraries can't afford to keep up? Imagine that at an individual level.
Misunderstanding #4: Some of our students genuinely can't afford these books. They and their families are taking out loans and maxing out credit cards just to get the students to college.
Misunderstanding #5: This is a misunderstanding on the part of our patrons: No, most college libraries - heck, most libraries in general - aren't in the business of buying and stocking textbook-y textbooks, but a lot of people think "libraries = books," and think we will have them.
All of this is to say that some students can't just buy their textbooks. So please, if a student comes to you and asks for a textbook, feel free to inform them that your library doesn't carry them, but keep the above misunderstandings in mind. Please try to help them find a copy through your consortium if your library belongs to one, and definitely have sympathy for students who can't afford/didn't know they need to buy textbooks. It's hard enough for students to come ask us for help in the first place; you showing them scorn (and it shows, even if you think it doesn't) will only make things worse.
One final note: I've talked to a few other librarians who do stock some textbooks in their libraries, both college/university level libraries as well as private high schools. I've been talking with some of my allies at my college, and I've decided to try an experiment next academic/fiscal year: I'm going to buy and keep in the library a few copies of the textbooks for each class with a high proportion of at risk students. If that works well, we'll expand. I'd love to hear from anyone who's got experience with this kind of program.