For me it's not about "oh man he bought this for 10 cents but sold it for 20 bucks, what an ass" but about the often obnoxious behavior of people who resell books at library book sales. I used to look forward to my library book sales so much it was almost like a holiday, now it's mostly me feeling anxious about hoping that someone who resells doesn't clear out the shelves I browse at before I can get inside (thanks to the lines that the resellers caused).