Who has not been impressed by the know-how of a vendor at a fair or a market ? Based on an observational study carried out at three fairs, the article describes the techniques used by fair demonstrators to sell « ingenious » products. To attract the customers, inspire them with confidence and conclude the sale the demonstrator must emit signs and create a relation between himself, his audience and the product. He also must be able to interpret the behaviour of potential customers and to respond to anything that might compromise his purpose, which implies a true mastery of the symbolic interaction.