Nate "Natron" Fasold has treasure-hunting in his blood. Back when he was a kid, his father, David, dove for Spanish galleons with famed treasure hunter Mel Fisher in Key West. "When I would see some of the Spanish treasure," he says, "I would imagine some of the sailors or pirates that have used them or worn them in the past."

But Fasold, a thin 42-year-old with a curly mop of black hair, now hunts peculiarly American game: legendary guitars. The walls of his 5-year-old store Black Book Guitars are lined with his treasures.

"When I see old guitars, I think of the players who used them and wrote records with them," he says. "That's what gets me excited: the story behind the guitar."

Back in 2015, he brokered the sale of Elliott Smith's Le Domino for $35,000. Smith used it to write his first solo record, 1994's Roman Candle, before it wound up in the possession of an ex-girlfriend. Fasold traveled to Boston and hand-delivered the guitar back to Portland.

In February, he auctioned a guitar that once belonged to Kurt Cobain for $80,000. The Hagstrom Blue Sparkle Deluxe still had Cobain's DNA on the fretboard when a Northwest musician—who got the guitar from Courtney Love and asked to remain anonymous—approached Fasold looking to sell it. He has plans to put a 12-string guitar—once played by R.E.M.'s Peter Buck—up for sale in August. Buck himself authenticated the instrument with a note in the hardcase.

Fasold, previously better known for dressing up as a hot dog to play with his one-man band Frank and the Furters, has developed a rep at his shop as a guy who knows how to sell musicians' valuable guitars.

Modest Mouse's Isaac Brock recently dropped off a one-of-a-kind Wicks guitar, which hangs behind glass on one of the shop's walls with an accompanying effects pedal. Scratch marks have already rubbed away part of the guitar's body, Fasold says, the result of "sweat under hot stage lights around the world."

Fasold is obsessed with verifying the authenticity of each guitar, sometimes traveling Pawn Stars style to avoid the "folklore" attached to some instruments. Before he'd sell that Cobain guitar at auction, Fasold showed it to former Nirvana guitar tech Earnie Bailey, who'd bought it for Cobain in 1992.

Later, a white Mosrite with an unusual logo underneath the pickguard sparked Fasold's interest. After some research, he unearthed its origin: It had been specially designed for Don Wilson of famed surf-rock group the Ventures in 1966.

Fasold traveled north to interview Wilson, 84, who was delighted to see his old instrument. "We played [this guitar] in Mexico City in 1966 and also in Japan the same year," reads a note in the guitar case. "Finding gems like this that have a history, a pedigree, is what we live for," Fasold says. "To actually see, play and own an item that made the music that's on a record puts it in another level."