a close friend deals in antique books;
as a private owner, getting a good price is difficult, and this has nothing to do with the value of the book itself.
book sellers have the means to make money, so they give you a pittance because they know you have no way to sell them yourself, and probably do not know the value anyway.. nor do the people who buy the books from the seller anyway.
it really is a matter of whether you need to get rid of them or not. if that is the case, call a few sellers, and get offers. ask them to point out the individual values, not just the bulk price.
otherwise, see if any seller will take your book to sell it for you at a price. this might earn you more money, but it will also take the longest time.. years, most likely.
and try to ebay them. always do that, make a suggestive post implying they look good and "mysterious" and are not just for specialists.