The minuscule pebble, encased in plastic and only 12mm by 9mm, has now been seized by the authorities and the putative vendor, identified as Alan Rosen of Florida, finds himself in no small amount of trouble.
The seizure has also visited sizeable embarrassment on Honduras, which was given the rock as a personal gift from President Richard Nixon a quarter of a century ago.
"This is the first time we are aware that moon rock was being sold illicitly on the US market," said Raphael Lopez, special agent in charge of the Miami Customs department. "The moon rock was offered for sale to Customs agents for $5 million".
The newspaper advertisement was placed as part of Operation Lunar Eclipse, which is a long-running undercover exercise involving agents from Nasa, the Post Office and US Customs. Its original intent was to squash an apparently thriving market in fake lunar items, including phony rock and bits of Nasa rockets.
This real piece of rock was apparently bought by Mr Rosen from a retired Honduran military officer in Honduras more than a year ago. It may be, however, that Mr Rosen's only crime is that he imported the piece into the United States without declaring to Customs. He could face smuggling charges.
The piece was originally collected by the last Apollo mission to the moon. Apollo 17 lifted off on 7 December 1972 - almost exactly 26 years ago. Its team of astronauts collected samples of rock that were dedicated to the people of the world.
The launch of Apollo 17 was witnessed by students from 78 different nations and the government of each country, including Honduras, received a piece on the rocket's return.
In the course of Operation Lunar Eclipse, agents were told that several of those governments had since sold their pieces of rock for prices ranging from $5m to $10m.
In his effort to ensure the authenticity of his purchase, Mr Rosen, 60, went so far as to contact the former commander of the Apollo 17 mission, Eugene Cernan, about a year ago.
"I found it very strange because there isn't anybody who's got a piece of the rock, including any of us who went to the moon," Mr Cernan said this week.
The rock was collected by Commander Cernan and his team in the Taurus Littrow Valley of the moon.
It is believed to be 3.9 billion years old. Randy Cook, a special investigator for Nasa, acknowledges that there may be several other chunks of legitimate lunar rock now on the market. "There are some moon rocks in private hands," he said.Reuse content