At the second hand book fair

Sometimes I come to these stalls and feel like asking the people who sell these books: - Would you like some of my books? I don't know what to do with many of them. They are also old, strange, 'curious' or even out of print. 
I even have a book-case fully occupied with the volumes of the Encyclopaedia Britannica that no one has consulted for many years. How much can you give me for it?

But the truth is that I don't say anything and I even buy one or two books!