NEW YORK - A US couple who tried to sell their five-month-old son on Craigslist have been arrested after a police sting operation by undercover officers.

John David Cain, 26, and Deanna Lynn Greer, 37, were taken into custody after trying to exchange the infant with undercover police for US$3,000 (S$4,100).

A resident of Elizabethton in Tennessee spotted the advertisement on Craigslist and tipped off the local police, who alerted Greene County Sheriff's Department and the Tennessee Bureau of Investigation.

Agents contacted Cain and Greer, and a meeting was arranged in the car park of a discount store, according to reports.

After the swap, the child was taken into protective custody.

"The baby's fine, it's with foster parents now," a police spokesman said.

Greene County Sheriff Pat Hankins said he had never encountered a case like it in his 34 years on the job, according to Fox News.

"Can you believe this? We bought a baby," he said. "It makes my heart drop."

The pair have been charged under Tennessee's "Hayley's Law", which made child abuse a Class A felony in the state after it was introduced in 2010.

It is named after a young girl who was found with her eyes swollen shut, ears bleeding and burns all over her body after being abused by her father and stepmother.

Cain and Greer are being held in jail on US$150,000 bonds .