German police are investigating a couple after they offered their eight-month-old son for sale on internet auction site eBay.

Renee Beck, a police spokesman in the Bavarian town of Krumbach west of Munich, said on Saturday the 23-year-old woman told them it was only a joke.

But he said police were nevertheless continuing their investigation and the baby was put in state custody.

"She says it was a joke," he said. "That's not yet clear. Detectives are investigating on suspicion of child trafficking."

A number of people called authorities across Germany after seeing the offer on eBay that read: "Baby -- collection only. Offer my nearly new baby for sale because it cries too much. Male, 70 cm long."

The opening bid was 1 euro ($1.57). There were no bidders during the two hours before the offer was removed, police said.

The mother was quoted in Bild newspaper saying: "It was only a joke. I just wanted to see if someone would make an offer. They've taken my son to hospital and I've got to take psychiatric tests next week."