Selling antique books can be profitable but requires some knowledge of the book market, some negotiating skills and a driving spirit of adventure. You can find antique books in garage and estate sales, or you may have inherited some books from a relative and want to try to turn them into some cash. As with any collectible transaction, there is an opportunity for profit or loss.
Research the value, as stated by condition of the book to see if what you have is in demand or just a nice curiosity. The books will be graded by condition: Very Fine (VF), Fine (F), Very Good (VG), Good (G) and Uncollected Conditions. The latter is a book too damaged to be worthy of sale.
Make a list of your book(s) and its condition and any pricing guide or information you can glean from your research. This can help you if you want to price the book(s) for sale. Search eBay to see if there are already books like yours for sale. You will be able to see if there is a market for them, and you might also see the price shifts or differences between the various quality levels of the books.
Contact an auction house. If your book(s) seem to be very rare you might want to have an auction house that specializes in antique or rare books sell them for you. It will take a percentage of the sale, but it can mean you will not be selling a Mona Lisa for the price of your child's finger paintings.
Visit antique stores. Most antique dealers will not have a problem looking at the book(s) and giving you a valuation.
Go to the library and ask the reference librarian how you might valuate the book(s). She will be able to direct you to sources to get a good idea of what the book(s) will be worth and possibly even know where you might be able to sell it. Many librarians are true book people and might have knowledge of someone who may be looking for that specific title.
On rare book blogs, people will be putting out their "wish lists" for others to see or comment on, and it might be a direct way to find a buyer for your book(s).