As concerns about the health hazards of vaping mount, a market for illicit cannabis-vaping products and the tools to create counterfeits is thriving online.

On Instagram, users offer products ranging from cannabis oils to vaping devices and packaging materials. On Amazon.com Inc., third-party sellers hawk empty packaging for vape products, and on Facebook Inc.’s Marketplace, sellers offer vaping products containing tetrahydrocannabinol, or THC, the mind-altering ingredient in cannabis.

These...