|>>|| No. 4866
>their listings for the same book as sold on different platforms can, again, vary wildly in price
I have direct experience of building pricing algorithms for book selling platforms. Very very few booksellers keep stock anymore1 - when they show you an "in-stock" price, they are actually reflecting the best price that their wholesalers will give them at the time - all booksellers use multiple wholesalers (and many of them share those same suppliers, but will have different deals on margin). When you order the book, they order the book from their suppliers, and "cross-dock" the book the next day when it arrives at their warehouse, or in some cases, their supplier will send it direct to you, with that retailers packaging. An additional quirk of pricing is whether the retailer includes postage in the cost of the book, as many do now.2
Price-comparison / book-searching sites will scrape or download those prices from retailers, regularly during the day - the prices will be cached for a while, depending on the site. Those retail prices will then change during the day, as stock changes at their back-end suppliers. Because a retailer has multiple suppliers, selling to them at different prices, the price of a book on any retail site could change multiple times throughout the day.
1 - Amazon obviously have the largest retail warehouses, but they'll only stock perhaps the top 100,000 titles, by sales volume, out of a universe of perhaps 20 million books in print.
2 - Because of the inclusion of the postage costs, many book-sellers will geolocate their users, figure out which country you're coming from, and modify the price accordingly depending on the postage cost. This also confuses the price comparison sites.