This topic has been raised before and essentially if you are an artist selling your work in Mexico then technically you are responsible for paying taxes on the income that you make. Now with that said I would suspect that very few of the foreign or local ex pat artists that live here or pass through San Carlos actually do pay any tax on any of the works that they sell.
Have heard that the artists selling their work say that they have always sold their work without paying taxes and without work permits! Maybe with the new immigration laws they will start checking this out. I know it hurts the legal gallery here in San Carlos and the Mexican art teachers and painters, like Hector Vari.