Amazon have a huge market advantage. There are other alternatives out there too. If you tend to buy a lot of second-hand books, the places to go are AddAll and Bookfinder - both do worldwide searches, and both include the Amazon providers and others who may do the same books but cheaper.
I've also recently joined BookMooch, which I hope will live long and prosper.
Here in Belgium, I flirted a few years back with Proxis, but in those days Amazon's rates were simply better. I may try Proxis again now.
Also, of course, we should support our local independent bookshops. For me, that means Sterling Books in Brussels, now that the American Book Center in Leuven has closed. Go on, pick up the phone and ask them to order that book you want. It will probably be no more expensive than Amazon would have been, and let's face it, the walk to the shop won't do you any harm. (Probably.)