It moves, in my experience, up the list, from bottom to top, wrapping around. Bear in mind it searches all the Amazons and only uses your preference when there are multiple hits. Even if, say, Amazon France is last in your list, if it's the only one with results, it's what you'll see.
This comes up when, say, a UK record store will somehow sell a CD with a US barcode and Delicious Library keeps pulling up the US release and confusing the heck out of the user.