Joseph G. Martin, who was accused of running the largest gold and silver fencing operation ever reported here, pleaded guilty yesterday in U.S. District Court to federal charges of racketeering and income tax evasion.

Martin, former proprietor of Royal Carpet and Tile at 1785 Florida Ave. NW and target of an undercover operation dubbed "Greenthumb," faces a maximum 30 years in prison and $36,000 in fines after pleading guilty to three of 51 counts.

The 37-year-old defendant had been charged along with Alan C. Danneman, 33, of masterminding a burglary ring that federal authorities estimated netted $8 million worth of stolen goods from Washington area homes. Danneman last week also pleaded guilty to racketeering and income tax evasion charges.

The two men purportedly had advised burglars which neighborhoods and homes to hit and what to steal and loaned them money to pay for bail and expenses between break-ins, investigators reported.

An investigation of the operation combined efforts of the FBI, the Internal Revenue Service and detectives from Washington and several other area jurisdictions.

After searching Martin's business premises and his Silver Spring home, federal authorities seized two Lincoln Continental and two Mercedes-Benz automobiles, rugs, silverware and jewelry, $205,000 in cash and nearly 9,000 silver dollars weighing more than 500 pounds.

"We're very, very happy with the way things turned out," said Assistant U.S. Attorney Joseph McSorley. "We set out to get the guys at the top, and that was done."

In attempting to strike a plea agreement with Martin, McSorley and fellow prosecutors staged a mock trial in open court in which the government laid out what McSorley termed the "overwhelming evidence" against Martin, including several secretly taped conversations.

Officials also said Greenthumb resulted in the convictions of 40 burglars who allegedly supplied Martin and Danneman, and in identification of property from about 800 burglaries in the Washington area.