Selling textbook on ebay, what kind of shipping bag to use?

Hi, I was thinking of selling my books on Ebay, but I'm not too sure what kinda shipping bags to use. I don't wanna buy the one from the post office because they cost so much more. I was thinking of getting those bubble mailers. Problem with that is that I don't know what size to get. So I was wondering if someone can tell me what a good size is for average textbooks.