What may be America's oldest silver dollar became the world's most expensive coin, with its owner saying it changed hands in a private transaction between coin collectors for nearly eight million modern day US dollars.

Steven L. Contursi, who owned the mint-condition 1794 Liberty dollar for the past seven years, confirmed he sold it to the Cardinal Collection Educational Foundation of California for 7.85 million US dollars.

The previous record price paid for a coin was 7.59 million US dollars for a US-minted 1933 20 US dollar gold piece, according to the American Numismatic Association.

The US began producing silver dollars in 1794, and this particular one remains in near-perfect condition 216 years later.

That being the case, the price it fetched was not surprising, said professional coin grader David Hall.

"Even if it looks like it's been run over by a truck it would still be worth a hundred grand," he said.

Part of the so-called flowing-hair silver dollars, the coin has a portrait of Lady Liberty with long, straight hair on the front and a noticeably skinny American eagle on the back.

"That's the type of piece that is available maybe once in a lifetime," said Martin Logies, curator of the Cardinal Collection, a nonprofit dedicated to preserving rare coins and educating the public about them.

He said the foundation plans to put the coin on display, just as Mr Contursi did much of the time he owned it.

Numismatic experts said it was among the first US silver dollars ever made.

"From the research I've done, it is unquestionably the earliest struck of all the pieces known to remain in existence," said Mr Logies, author of The Flowing Hair Silver Dollars of 1794.

Of the approximately 1,750 such dollars produced that year, only about 150 are known to exist.