When I started seriously reading books, not ebooks or just pdfs, I mostly went to amazon.com for checking book prices. I found out it is not always cheapest way to purchase a desired book there. I'll share here list what webpages I check when I look for a book (new or used)
Amazon [amazon.co.uk]
I prefer british version .co.uk because delivery is faster to Europe. I do not want to wait one month. A lot of sellers have their own webpages, but prices are sometimes different, as an example, I bought book on Amazon through BookDepository, which was cheaper with shipping, compare to the price on their own website.
Alibris [alibris.co.uk]
Again british version, .com version exists as well. They are more expensive considering 11 euros for shipping to Europe. I used it only once so far, book got lost, a replacement is on its way. I hope this one arrives to my location.
AbeBooks [abebooks]
I consider this to be USA based, most of sellers I checked, were from US. Many are from India with their international editions. Possible to find good bargain. Shipping varies with sellers, I always compare, sometimes more expensive book is cheaper combined with a shipping.
If you have any other tips, please share !