Nowadays we have to face a new types of communication including electronic way of communication. It’s natural, normal in our life that we use an Internet to call to th friends or to buy things via auction web sites. And we have to consider if classic law regulation fit to this new type of communication, if they are still functional. It’s especially important when we want to avoid consequences of our declaration of will. Often, in this situation we use regulation about defect in a declaration of will. And this is the main idea of this article, especially that the base of it is connected with real life (sale of a car for extraordinary price). It focuses on problems connected with classic and electronic market and tries to answer the question if classic law regulation are able to be used in modern types of communication.