That could be a lot of research. Given the nature of the books I tend to buy (usually no single item over $15-20, but in bulk orders up to $400 sometimes), I have little incentive to spend much time researching for "good deals" across multiple sites, possibly making multiple orders instead of a single order.

I'm sure if I spent an hour or two finding the lowest price on 20-30 items I could save a bit of money on the order(s), but I don't think it's really worth my time for my default use case.

Now, if I were looking at items like text books or reference books, I would likely do the research. Such materials are often sold at huge variation in price across retailers ...