1. Amazon Marketplace
Probably the online merchant users will be most familiar with, Amazon has a large Marketplace of used books being offered by third party sellers (used bookstores and individuals alike) that can be used to get a general idea of what the current market value for your used books are. Simply locate your specific edition and then click on the link to get used pricing.
When the list of copies appears, use the limiters on the side menu to select the one that matches the condition of your copy.
AbeBooks.com went live in 1996 and is now one of the largest international marketplaces for used books, allowing users to “search books, fine art & collectibles offered by independent sellers around the world.” Like Amazon, it offers the ability to narrow its searches to used/new in the left hand menu.
However, it does not further narrow by specific condition.
A network of independent sellers that also launched in 1996, Alibris has additional filters that allow for narrowing results down not just by new/used, but also by specific condition and rarity. Once you find the edition of the book that you are looking for, click on the link to open the title's record and you will see additional filters that include the ability to narrow your list by condition:
Ebay doesn’t specialize in books the way that other marketplaces listed here do, but it offers one very useful feature: the ability to see what items have sold for as opposed to just what people are asking for them. Click on the advanced option to the right of the search box and then select “sold listings” to view these items.
Zubal is truly a rare book seller. They will buy books or collections. John Zubal, the owner, the owner is very approachable. His prices are more realistic than at other sites since they are set by just this company. The only issue with Zubal is that it is just rare and valuable books. You will not find a lot of popular content unless there is some value associated with it. His page does include links to other buyers/sellers and some interesting news articles about the used/rare book trade.
5. BWB – Better World Books
BWB is a slightly different type of seller and buyer of books. They have a mission of charity and those that donate have the privilege of knowing that some of the books or profits from the sale of the books that they donate will go to help schools in other countries get books for their students. That said, this organization gets a large number of titles and puts most of them for sale on the web to support its programs. BWB will pay for shipping to get your books and they do post guidelines to what type of content they will take. Prices are a bit more modest but the selection is very broad.