The decoys went to 10 different stores Wednesday and were all able to purchase the materials, police said.
Officers met with management at the businesses after the sting and presented them with a letter that explained the city's municipal code, which states that it is a crime for someone to sell or furnish any graffiti-related items to a minor.
Businesses are also required to post a sign warning about graffiti in a conspicuous place on their premises, but no signs were observed at any of the businesses, according to police.
Petaluma police say they have had 218 reported cases of graffiti this year, as well as many more unreported cases. The cost to the city and businesses for paint, cleanup and labor is about $250,000 annually, police said.
Police say that more sting operations are planned.