Think of this logically.  You are a vendor, your business is selling your products, and your goal is to be as prosperous as possible.  Your first difficulty is getting customers.  The easiest part of the solution is making it as easy as possible for potential customers to find you; thus signs and maps.  Of course, that doesn\'t discount the possibility of hidden vendors.  Say a random, average looking peasant, in an average looking dwelling, happens to sell spells at a lower rate than the greedier vendors.  Something along those lines.