The solution is to make more money so that a 50% off sale doesn’t tempt you. If you think about it, if the books were really good, they wouldn’t be on sale.
Plus, I don’t think it’s good to always want a discount. We all like a bargain but sometimes the reason is so that we can buy more stuff with the remaining money. And it’s a type of gluttony to keep wanting more stuff. Also, it can be indicative of a person who doesn’t know his/her priorities. They want a bit of everything instead of a group of things that they really desire. So for instance, I don’t go to Walmart and places like that because they encourage purchasing many bad products for less versus a few good products with quality.
Also, if you think about it from the proprietor’s point of view, she or he would have liked the full profit margin but is selling the books because s/he has to admit nobody is really buying them and s/he needs to free up the cash to buy something that will sell. But if you really wanted those books, you would have bought them before they went on sale, and been more helpful to the proprietor.
But it depends on your own philosophy. Say you always give people a deal, it might be alright to expect them to always give you a deal.