He decided on a test, and added books to his baskets on both Amazon.com and Amazon.co.uk. He says:
On Dec. 15, I checked my shopping baskets. Nine of the U.S. books had increased in price; three had decreased. At the British branch, nine had increased and none had decreased. The increases were modest — often about 5%, sometimes less. But ... they were perplexing.The Amazon spokesman Streitfeld spoke to wouldn't talk about the company's pricing strategies, while an e-commerce analyst he spoke to speculates that Amazon may be using a dynamic pricing policy which involves selling identical material for different amounts based on the customer's willingness to pay.
But this earlier article by in The Inquirer may hold the answer. Amazon prices are based on sales rank and once you place the items in your shopping cart, only hold good for an hour.
You may remember that a few years back Amazon ran into trouble with consumers over the pricing of DVD's.