BIG SALE on things you don't want

I recently bought headphones from the student union, thinking the 20% sale applied to everything. I ended up paying more than what the headphones were worth. I hate sales where they say there's a big discount going on, but when you get inside it's only on the things you don't like. I really hated it when I found my sister had the same headphones. She supposedly bought it before I did and paid less. Oh well..no more shopping at the student store for me.