The expertise of the dealers (along with the presentation and the surroundings) is one of the things that differentiate a book fair from a book sale. At a fair you can talk to the dealers about the books you are interested in. They may have titles you want in stock at home, they may be able to advise of another dealer who specializes in that field, they may remember your interests and offer relevant books to you at the next fair. They may equally be interested in buying books you no longer require. Also many of the dealers offer a book search service.