The tintype, dating to 1878, depicts Billy the Kid and several members of his gang, the Regulators, playing croquet outside a Lincoln County, New Mexico, building, according to Kagin's Inc, the company that authenticated the photo and is handling its sale.
Billy the Kid -- also known as Henry McCarty and William Bonney -- is one of the best-known figures of the American West, simultaneously a folk hero and an outlaw credited with at least eight murders.
The photo was apparently taken after a wedding, the company said.
Guijarro had earlier told CNN affiliate KSEE in August that purchasing the photo was a last-minute decision.
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"I had just a couple of bucks left, and I found three photographs I liked, and of the three, the oldest tintype, I actually kind of chucked it back in the box," he told the station.
Since, the photo has been authenticated by a photography expert and a facial recognition expert who has reportedly done work for the National Security Agency and the FBI, according to KSEE.
"The historical importance of a photograph of Billy the Kid alongside known members of his gang and prominent Lincoln County citizens is incalculable. This is perhaps the single most compelling piece of Western Americana that we have ever seen," Kagin's owner Donald Kagin said in a statement on the company's website.
The only other known 'Billy the Kid' photo sold for USD 2.3 million in 2011.