Long Melford Book Fair
Each stall will be run by a different dealer, some of whom may specialize in one field, e.g. first edition fiction or aviation, others may bring a general selection of subjects.
Thus the customer gets to view a wide range of books without having to go from bookshop to bookshop. 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.