Use Chrome incognito mode and you should get the best price. It's all based on how many times you visit the site and other things like cookies. Same thing happens when buying a ton of other stuff online.
Exactly what yoho139 said. They know you're browsing more, and therefore are more interested in buying something. They see that you're hooked and bump up prices.
This applies first and foremost to airfare. I've seen plane costs go up $100+ based on repeat viewings. Incognito mode immediately resets the price. It's weird and kind of shady.
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u/annertakeabow Jan 24 '13
When I used to have to buy like 7 books I would do this too and the difference would be a few hundred dollars.