It's worrying how much influence Amazon exerts over book selling online. In the UK, Amazon have pretty much conquered online book sales. For antiquarian book sales, I can't think of anyone else as alternative online rival to AbeBooks (eBay?)

If you're buying books through Amazon, consider purchasing from Amazon sellers. If you're in the UK, try Wordery. (They are based in the UK but offer free worldwide delivery - much like The Book Depository before them which is now owned by Amazon.)