Know exactly what you want. To find a rare book at a fair price, you should know everything about the book you want before you begin searching for it. Know, for example, the different editions and printings of the book, unusual instances of the book, the different publishers and any other detail that might change the value of the book.
Check rare book trading sites online. The Internet has brought on a revolution in the rare book market, so take advantage of it. Amazon, Biblio and Alibris are excellent places to start. Use these sites not only to shop for the book but also to gather information about it.
Learn the language of book conditions and sizes. Before you buy a rare book, you need to know the basic terms of book buying. Words like "Mint," "Excellent" and "Good" have very specific meanings when referring to the condition of a book. Similarly, the type of binding and size of the book affect the value of a book and also have specific terminology. Read a good introduction to rare books at The Vintage Bookstore to learn more.
Contact book sellers directly. Many rare book sellers are experts, so getting in touch with them will certainly help you find a book if you are having trouble. And most rare book sellers simply enjoy talking about books, so have a conversation with an antiquarian or rare book dealer to learn more about the book you're searching for.
Take a look at the book or at detailed photos before you buy. Signs of damage from mold or, worse, insects can destroy a book's value. So check for yourself before you commit to buying a rare book.