(Reuters) - A Vermont thrift store employee pleaded not guilty on Monday to stealing letters and holiday cards written by the poet Robert Frost and selling them for $20,000, police said.

Timothy Bernaby was arraigned on grand larceny charges in White River Junction, Vt., about 100 miles north of Frost's grave in Bennington, Vt., which is marked with the epitaph "I had a lover's quarrel with the world."

A donor mistakenly dropped off the memorabilia, including Frost's hand-written Christmas cards and other correspondence, to a Listen Community Services thrift store in White River Junction where Bernaby worked, police said.

Bernaby, 42, of Hartland, Vermont, sold the papers for $20,000, police said.

Authorities are investigating the buyer of the documents, who may be linked to an illegal memorabilia trading network, Detective Michael Tkac said.

The author of "Stopping by Woods on a Snowy Evening" and "Mending Wall," Frost lived much of his life in Massachusetts and New Hampshire and is buried in the tiny Vermont graveyard.

Frost memorabilia often fetches high prices at auction. A 1941 Christmas card signed by Frost and containing the three-stanza poem "I Could Give All to Time," is being offered on eBay for $5,999 by a Las Vegas-based dealer.

(Editing By Barbara Goldberg and Daniel Trotta)

Mochila insert follows ...