A veteran archivist employed by the New York State Library has been arrested for stealing hundreds of historical documents from the state government and selling them on eBay, Attorney General Cuomo announced yesterday.

The archivist, Daniel Lorello of Rensselaer, who worked at the library for nearly 30 years, confessed to smuggling out hundreds of rare documents starting in 2002, before a former history professor in Virginia noticed a suspicious letter from John C. Calhoun for sale on eBay this month and notified New York authorities.

Speaking at the announcement of the arrest yesterday in Manhattan, the professor, Joseph Romito, 59, and Mr. Cuomo explained how a mixture of historical and cyber know-how helped them track down an alleged thief who mastered both worlds.

Mr. Romito, who used to teach medieval history at the University of Illinois, is also a Calhoun buff, and said he was browsing for memorabilia of the former vice president on eBay on a whim on January 16.

He said he was attracted by a listing for the Calhoun letter because of its political content: The letter was sent in 1823 to the personal secretary of a former New York governor, DeWitt Clinton, asking for Clinton’s support as Calhoun considered running in the 1824 presidential campaign. Before bidding on it, Mr. Romito said he first checked his 28-volume set of Calhoun’s collected papers, where he found the letter listed in the index. Under a photo of the letter, a yellowed document scrawled with brown ink, Mr. Romito said he noticed that it had been archived in the New York State Library.

“I thought, ‘Why would the library release it?'” Mr. Romito, now a practicing attorney, said.

The next day, he called the library, prompting librarians to search for the archived document. They discovered that it was missing.

A week later, as the document went up for auction with a starting bid of $250, both Mr. Romito and the attorney general’s office — which the librarians had called — began bidding in an effort to save the document from being sold away.

With neither realizing the other was bidding, the auction ended with a final bid of $1,802.77 by the attorney general’s office, a few dollars more than Mr. Romito’s last offer.

The seller information listed the name and address of Mr. Lorello, an archivist at the state library who had full access to the institution’s historical archives. After obtaining a search warrant on January 24, state investigators found hundreds of other documents stored in more than a dozen boxes at Mr. Lorello’s house and obtained a written confession from him that day.

According to the confession, Mr. Lorello said he used the money to pay for renovations to his house and his daughter’s credit card bill. “No one at work ever suspected anything or challenged me at any time,” he said.

Mr. Lorello said he “particularly liked items associated with the Revolutionary War, Civil War, Mexican War, black Americana, World War I, anything related to the Roosevelts, and Jewish items.” The most expensive item he stole was an 1835 Davy Crockett’s Almanac from Tennessee, which he sold for $3,200 to a collector in Colorado.

The commissioner for the state Department of Education, which oversees the library, Richard Mills, said the department would investigate how Mr. Lorello carried out his thefts and institute new security measures.

Mr. Lorello is facing several felony charges, including grand larceny and scheming to defraud.