I have been an online bookseller full-time since 2000.
Here are a couple of points about ordering used books online:
The largest used bookselling site, with over 13,000 dealers, is www.abebooks.com.
The Abebooks sites for particular countries, e.g. www.abebooks.co.uk, www.abebooks.de, all list the same books from the same dealers, but with prices shown in the local currency, i.e. it doesn't make much difference which abebooks "portal" you go through.
While for the general public Amazon is probably a more well known site than Abebooks, the range is actually larger on Abebooks. This had led to one anomaly that it is worth being aware of. There are a few large sellers on Amazon who do not actually have a physical copy of the book, but are using software to copy titles that are available on other sites such as Abebooks. For example, a book may be listed at a fairly high price such as US$300 by a seller on Amazon. When they receive an order for that book, they then go to Abebooks where a copy of the book may be much cheaper, say $100, and buy that cheaper copy to fulfil your order. (The Amazon seller may have the book sent to them to send to the customer, or they may have the Abebooks dealer send the book directly to the customer). So, if you find a used book that seems to be only available at a very high price on Amazon, it is worth at least checking on Abebooks first for a cheaper copy.
There are a few other meta-sites that search all the other bookselling sites, which can sometimes let you find a book that is not on Amazon or Abebooks.
These sites include:
www.vialibri.net
used.addall.com
www.bookfinder.com
These sites will amalgamate results found from the main used bookselling sites, such as:
www.abebooks.com
www.alibris.com
www.biblio.com
+ several other sites.
www.bookfinder.com is now owned by Abebooks, but runs independently without any behind-the-scenes manipulation to drive people to Abebooks.
In much the same way, Abebooks is also now owned by Amazon, but is managed independently in much the same way as it had been running before Amazon bought it.
Abebooks is a particularly good site for its "Wants" service. If you sign up as a customer, you are able to leave a want for a book that you can't currently find, or that you can't currently find at a reasonable price, and you will be emailed automatically when a seller lists a copy of that book.
One downside of Abebooks in recent years is the proliferation of "PODs" or print-on-demand reprints of out-of-copyright books. These are generally not so nice to own as the original hardback editions. Sometimes the illustrations are not reproduced as clearly. Or they just not as a nice in terms of "nostalgia" values, or their value as a "collectable", as earlier printings. It can take some time to become familiar with what is a recent POD reprint of an older book, particularly when sometimes the bookseller will use the original date of the book, rather than the date it was or will be printed, in their description (which is against the site's policies, but does happen sometimes).
The basic principal that determines prices books are listed at and will sell for is supply vs. demand. This is why a cheaply made paperback that did not cost much when new can sometimes ending up costing $50 or $100 or more, while a book that was originally published as a lavish expensive hardcover may end up never selling for more than its original cost, if the number of copies available is more than the number of people looking for it.
The listing sites also show some strange, excessively high prices however. This sometimes does not represent what a book is actually worth or likely to sell for, but is instead a product or artifact of the listing software used by some sellers. Some sellers price each book one by one, with a human working out a price. Some other sellers however use pricing software that follows algorhthyms, which sometimes produce absurd results. For example, if there are only 2 copies of 1 book from two different sellers, and they are both using pricing software that sets their price to be $0.50 higher than the other copy, then the prices of the two books will both start climbing skyward. They may start off at $3.00 and $3.50, and end up at $900 and $900.50. This does not reflect the real value of the book, since if a third seller comes along and lists a copy for $25, the pricing software used by the other two sellers will then pick up that new price in its next trawl and return the prices of the expensive books to $25.50.
Another site sometimes of interest is www.isbndb.com. This free site lets you enter the ISBN of a particular books, and then you can look at the price history over time on different sites, including abebooks, alibris, amazon and other sites. It is worth remembering though that the prices shown though are the asking prices, and not necessarily prices that copies actually sold at.
