An on-line invitation to buy stolen computer data led to the arrests of six computer users and the seizure of more than 20 computer systems, the U.S. Secret Service announced today.

During an eight-month investigation based in New Jersey, an undercover Secret Service agent using the name "Carder One" and a confidential informant operated a private computer bulletin board system called Celco 51, authorities said.

"What we really did was set up an old-fashioned swag shop' in cyberspace," said Assistant U.S. Attorney Donna A. Krappa, who is prosecuting the case. A swag shop is an establishment where stolen goods are traded.

Agents have arrested suspects in California, Michigan, Texas and Brooklyn, N.Y. They also grabbed computer hardware and data in those states as well as in Alaska, Connecticut, New Jersey and Virginia.

A bulletin board is an electronic meeting place where computer users can exchange information. During the probe, officers identified people who broke into computers and stole information to get free cellular phone service, authorities said.

Over the Internet, the undercover team advertised that Celco 51 catered to those involved with all aspects of computer fraud, court papers said.

The cellular phone fraud targeted in this case is usually accomplished by stealing electronic serial numbers and mobile identification numbers, authorities said.

A pair of active numbers can be programmed into a cellular phone to make it operational. Calls made on the "cloned" phone are charged to the legitimate user, who only discovers the fraud when checking the monthly bill.

If convicted, the defendants face maximum possible sentences ranging from five to 15 years, depending on the charges.