Where I'm from (Ireland/UK) students buy their own books, but they're much much cheaper than the US. I think the price of grade school textbooks is high because it's school districts doing the buying. College textbooks are also a lot cheaper, eg I'd expect to pay about $30-40 for a typical textbook from a major publisher like Pearson or similar. I got a hell of a shock when I moved to the US and found college-level textbooks to be 3-4 times more expensive than what I was used to.
Since both college and grade school texts are more expensive in the US, as you report, it seems more likely that there is a common cause for the difference in book prices rather than the price difference for grade school texts being due to school districts buying them.
My (under-researched) theory is that by the time people get to college, they've already been acclimatized to the idea that textbooks should be very expensive by 12 years of grade school, so most of them passively accept the egregiously high prices.
Part of my interest in this stems from my surprise at learning that my wife saved a few thousand $ by buying almost all her college textbooks from the UK, despite the high shipping fees. Too bad this wasn't an option for law books :-/
US college students don't "passively accept the egregiously high prices." Most are shocked about the prices, and there is no acclimatization to the idea that books "should be very expensive."
One reason for the high price is that the text book publishers have a short revision cycle. If the book chapters and page numbers change a bit every couple of years, and especially if the problem sets change, then there's little resale value.
Another is the bundling of workbook material (just like I pointed out for Ireland), even though most instructors don't use that material. This also makes it harder to resell the book.
As it says: "What makes students’ textbook purchases even less responsive to price increases than any other commodities is that many faculty members choose and assign their textbooks with little regard to the cost of the textbooks"
As I pointed out, the increase in freedom of choice by Irish instructors caused similar reductions in resale value of primary and secondary school text books in that country.
