The story of how Keith Haring came to paint a crib began on a quiet, ordinary afternoon in 1986. His best friend’s wife was pregnant, and the couple didn’t have the money to buy a new crib for their home in New York City’s Greenpoint neighborhood. “I called my parents to ask if my old crib was still in the attic,” says artist Kermit Oswald, Haring’s friend since childhood. “I got it and I painted it yellow, then Keith came over, we had a few beers and he painted the rest of it.”

Haring is famed as an enduring, globally recognized celebrant of Aids activism, nightlife and the New York Bohemian scene of the 1980s. But he honored his connection with his straight best friend even as he rubbed shoulders with Andy Warhol, Robert Mapplethorpe and Jean-Michel Basquiat.

Oswald’s collection of Haring’s work is now on display in Haring’s House: Works From the Collection of Kermit Oswald, a public exhibition at Sotheby’s New York this month, with the works going to auction in two sales on 14 and 15 May. The most surprising item is the crib. It’s taxi-cab yellow with paintings of dachshunds (the Oswald family dog) and two figures representing Oswald and his wife, Lisa.

It’s one of 20 Haring works going to auction. The marquee lot is a 1985 self-portrait, one of only six the artist ever painted on canvas. The auction estimates are $3m-$5m, with the crib alone estimated at $250,000 to $350,000. With many items never seen publicly, it shows a personal side to one of the most iconic gay artists in history.

Oswald knew Haring since they were five years old. They met by passing notes at church in their home town of Kutztown, Pennsylvania. As boys, they shared a passion for “creating things” and started drawing together. “We rode bicycles and played baseball, though on different teams, and were always in each other’s homes,” he says. They delivered their paper routes together; the loser of the morning race had to buy ice-cream.

Haring loved “artistic pranks”, Oswald says. In high school, a neighboring farm’s silo had fallen and was lying across the field. “Keith said it looked like a blimp,” Oswald recalled, so at night they painted the Goodyear logo across the side. “This was farm country. There was no graffiti anywhere, and Keith did not fundamentally see himself as a graffiti artist.”

Haring’s early untitled works on paper from the late 1970s and early 1980s often used cut-up newspapers and Xeroxed clippings, a practice influenced by William Burroughs’s cut-up technique. The two met in 1983 and collaborated on Apocalypse, a series of silkscreen prints with text by Burroughs, in 1988, the year Haring tested positive for HIV.

In our conversation, Oswald offers a clue to interpreting Haring’s oeuvre: “You have to remember, Keith was a paperboy.” Oswald says the artist’s many untitled pieces should be read alongside the date. “News headlines were always a big part of Keith’s life,” he says. “Works were untitled because he wanted you to see the date and look at the headlines from the New York Post or New York Times.”

When asked what Haring would be like now, he says: “It was a more analog world then. If you wanted to comment on the news, you had to put ink on paper. In today’s lexicon, Keith would be a blogger and definitely a news junkie.”

They both moved to New York in 1978 to study at the School of Visual Arts. Oswald built the workshop in Haring’s studio, every frame Haring used and installed Haring’s exhibitions. Oswald’s father was a carpenter, and Oswald taught Haring how to work with wood. The wood relief pieces in the sale, including an untitled 1980s work depicting a figure falling down a flight of steps (believed to be Haring’s earliest use of the “staircase” motif, common in his work), are estimated between $150,000 to $200,000 and could not have existed without Oswald’s help.

By 1985, Haring was famous and painted his self-portrait from a Polaroid, his face on the body of a sphinx. He invited Oswald to the studio and told him to take any work he wanted. Oswald chose that one.

The pair called Haring’s mounting fame “the tiger”. “You work and work and work thinking you can finally climb on the tiger and ride it,” Oswald says. “But once it actually happens, you just have to hold on for dear life, and the best you can do is hope to steer it a bit. The tiger, you can’t handle it. The tiger takes you for a ride.”

Oswald chose not to ride the tiger. “This wasn’t a battle. There was no competition. You don’t compete against your friends.” He repeats a few times, unprompted: “No competition. As Keith got more successful, he bought me designer clothing for openings to make sure I looked good.”

Oswald also learned Haring was gay in the course of their friendship when they were 16 or 17. “When you love someone, you love all of them, their heart and soul, so I never questioned my love for Keith when I learned he was gay.”

He liked Haring’s longtime boyfriend, Juan Dubose, who was a DJ, and tells me Haring was inspired by house and new wave music of the time (in particular, Oswald says, Haring was a big Talking Heads fan). Oswald has hundreds of mix tapes from Haring during those years. Oswald’s son is also gay and, incidentally, Haring is his son’s godfather. Oswald believes Haring “was preparing me for something that was going to be a big part of my life”.

Haring died of Aids-related complications at 31 years old on 16 February 1990. “I was with him the day before he died. He passed the day after my birthday, which still sometimes makes the day hard.” Earlier, Oswald drove Haring’s parents to the city when it became clear “that it was time, that things were getting bad”.

He says: “I sat his family down and told them that Keith was HIV positive.” Haring was too sick to do it.

Speaking on why he’s selling items from such a long, intimate friendship, Oswald says: “I came to the realization that my relationship with these works is experiential. The work is just the physical result of it. I love the memory of being in the room when most of them were being done.”

Still, many would struggle to part with work of such an iconic artist, and of a friend. But I get the sense Oswald wants the work to end up in a museum, where it can be seen by others, though he never states this explicitly: “You don’t own art, you are just a custodian of it.” He says the “real purpose” of these works is “to be out in the world to do what they’re supposed to do”,

The lingering popularity of Haring and his market demand, even 35 years later, is something Oswald seems reluctant to comment on. Kathleen Hart, the Sotheby’s specialist on the sale, tells me the Haring market remains “very strong”, with “a premium for objects that are fresh”.

Oswald laughs. “A lot of it was pieces of wood we found on the street. It was refuse. We were dumpster diving!” He adds: “There’s nothing precious about art, especially Keith’s, until you make it precious. To take something that’s nothing and turn it into something precious, that is magic, and not many of us can do that. Keith could.” In contrast, Jeffrey Deitch, the longtime art dealer who represents the Haring estate and co-authored a 2014 Rizzoli monograph on the artist, hopes a young family buys the crib. “I hope it ends up in a nursery!” he says. “It’s for children. It should be used.”

If these materials do not exactly reveal a new Keith Haring, they at least help to give a more rounded, human picture of his life. Speaking on gentrified New York and today’s art world, Oswald muses: “I think New York would be different if he had lived, if all of them had lived. When I think about all those artists and dancers and musicians, if they had stayed, New York would be such a different place.”

Oswald still paints, mostly watercolors, but his real art now, he says, is gardening and family. “I didn’t want the tiger to pollute my love for art,” he says. He likes that interest in Haring endures. “As a society, we rarely have intimate contact with a great artist. Most of the time, we just have their work, and when you finally meet them, it’s disappointing. But Haring is the opposite. Knowing him better helps you understand his work. And I can tell you he was a generous person, a great man and a great friend.”

• Haring’s House: Works From the Collection of Kermit Oswald is on show at Sotheby’s New York until 14 May