a. I see a 50% off book at O'Reilly that I'm interested in. b. I check the price at Amazon.com. The regular price is usually a few dollars more than the discount price at oreilly.com. c. I purchase the book at Amazon because it is just easier, even if it slightly more money.
I actually feel a bit bad about this: I'd rather give the money to Tim and Co directly. And, yes, I know it isn't that hard to do this from the O'Reilly site...