Amazon....
Not sure how they set their prices.... The website displays new/used books by title, side by side. Even rates the condition of each and the volume available. The price on a particular title can vary over time (maybe depending on inventory). They have to have a large amount of easily accessed info available to set prices.
From my limited number of purchases from them, pricing almost seems like an auction. The price is set and allowed to drift until a bid is received....
"America is a country which produces citizens who will cross the ocean to fight for democracy but won't cross the street to vote."
Author unknown
Bookmarks