Investigators found sellers on social media and pretended to be buyers.
So far, they have confiscated 300 pounds of illegal fireworks, worth thousands of dollars. Four people have been arrested.
RELATED: Reward money of $2,500 for reporting illegal fireworks in East Bay
The Contra Costa County Fire Marshall says a fire earlier this month shows what can happen.
"A fire that occurred in Antioch, where an apartment complex ended up burning down displacing 40 residents from just a single firework going off," explained Fire Marshall Chris Bachman.
One person was arrested in that fire.
The department is asking people to phone-in tips anonymously.
VIDEO: Fears of fireworks igniting fires in Bay Area before July 4th
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