it depends on what kind of guild we are talking about. Are we talking about craftmanship guilds? Mercenary guilds and so forth. As for craftmanship guilds, they are more regulatory bodies. They dictate the quality of the products one can sell, as well as who can even sell those products, and to whom you are even allowed to sell, + set up prices. To keep it short, they are kind of monopolies.
Depending on how your fantasy world is built, and how the guilds are (for example, in my case local and kingdom guilds coexist), guilds could for example act as "Fantasy amazon one-day delivery, which wouldn't be uncommon if they have a wide network).
The dynamics between kingdom guilds and local guilds could be even more interesting since both might have clashing interests, for example.