With 18th c. books, there are typically a few issues that will hurt or eliminate value. As always, there are exceptions, but here are some of the common types of books and issues that cause an 18th c. to be of less value, or no value:
1: Incomplete sets or incomplete books. It is not uncommon to find single volumes from larger works. The title-page will typically indicate if the book you are holding is one volume out of a larger set. For example, if you have a book that has "Tome Second" printed on the title-page, then you have the 2nd volume out of a larger work. Odd volumes like this are typically not worth very much to the collector, though they may have some meager value. The other common problem for these books is that they may be lacking a text-page or an illustration. The leaves may have been removed, or the binder may never have bound them in. A missing leaf can turn a $1,000 book into a $100 book; or, it can turn a $100 book into a worthless book.
2: Modern, utilitarian bindings. Often, we will see books that have been rebound in either an amateur binding or a library cloth binding. This will always impact value. If you have a book that would normally be worth $300, bound in library cloth it might fetch $50, for example. The more original and well-preserved the book is, the more value it will have to the collector.
3: Various issues with condition. Let's say the book is complete, but it has water-stains, the spine is chipped, the boards are detached, a rat has chewed the binding, the paper is heavily toned... these things are somewhat common but are important to value. This is why photos should always be submitted. Condition plays a major part in a book's value.
4: Common subjects. The more common the subject, the less valuable the book. For example, Biblical commentaries and theological works were among the most widely printed types of books. Another example would be law books - many were printed. These types of books tend to be lower on the value-scale ($100 to $200); however, there are plenty of exceptions, so it is important to have us evaluate them regardless.
If your book was printed prior to 1800, then it is always advised to submit it for evaluation. Simply email webuyoldbooks@hotmail.com with a photo of the title-page and the binding, and indicate in your email the number of volumes present (if more than one.)