But Keith Grabowsky, who the FCC said posted an ad on Philadelphia Craigslist, may have known something was up with the device he had for sale for $300.

"Because of the nature of this item, few details are given out. This is not a toy. For adult use only. I just want to get rid of it as fast as possible," the ad said.

Craigslist has gotten into legal snarls previously over certain kinds of ads on its sprawling, multifaceted sites. A 2009 lawsuit against the company by Cook County, Illinois, for allegedly hosting ads for prostitution, was after a federal judge said Craigslist had done no wrong. If the site bans illegal activity, warns users against it and removes inappropriate content, it's not responsible for users ignoring those rules, the judge said.

In this case, Craigslist specifically bans users selling or advertising signal jamming devices through its site. "Therefore, you have not only violated federal law, but also contravened the contractual agreement that governs your use of Craigslist," the FCC wrote in its citations. The agency said it would contact Craigslist about the violations. Craigslist declined to immediately comment.

Also on Monday, the FCC issued a "" in English, Spanish and Mandarin warning consumers that using signal jammers is illegal.