James Micioni was born and died in the same place -- a modest, two-story house on a hilly neighborhood in Boonton, New Jersey. He never married, never became a father and never owned a car. He walked to nearby jobs as a high school custodian and a chemical-factory worker, leaving his small, working-class town only when called to serve in Europe for the last three years of World War II. He was a die-hard fan of the New York Yankees, but also of Jackie Robinson. And he spent most of his life curating a treasure trove of baseball cards that experts believe to be one of the most extraordinary private collections in the hobby's history.

Micioni -- "Uncle Jimmy" to practically everyone who knew him -- died at the age of 97 on March 8 and left his stunning collection to seven nieces and nephews. Most of it will be sold at auction over the next few months. There are sets from the 1930s and the mid-20th century in pristine condition, and there are dozens of autographed cards from the likes of Babe Ruth, Lou Gehrig, Jimmie Foxx and Pete Rose.

It has all been separated into 2,000 lots that will be made available by Wheatland Auction Services through three different auctions, the first of which will begin at 7 p.m. ET on Sunday and span four weeks. Chuck Whisman, who owns the company alongside his wife, called it "a once-in-a-lifetime collection."

"There's a saying in the hobby that collectors are temporary custodians of history, and I think Uncle Jimmy was a prime example of that," said Joe Orlando, president and CEO of Collectors Universe. "If you look at the type of collection that he assembled, it required a lot of work. This wasn't a mere find of uncirculated baseball cards; this was a collection that needed to be assembled in a meticulous manner."

Micioni used to mail baseball cards to teams hoping for autographs from their star players, keeping a ledger that tracked every item he sent out. Shortly after his death, his nieces and nephews ventured into Micioni's attic and found binders separated by decade and packed with autographs, including six Ruth cards from the famous 1933 Goudey set. Orlando, who helped to officially grade the cards for Professional Sports Authenticator, estimates that those half-dozen Ruth cards together are worth up to $1 million in total.

"This," Orlando said, "is evidence that buried treasures still exist."

Micioni's family knew all about his passion for baseball memorabilia. His seven nieces and nephews, who now range in age from 49 to 69, would be gifted parts of his collection for birthdays and on Christmas through most of their lives. Over the past 15 years, they would stop by his house on a weekly basis to help him run errands and would usually have a box of cards waiting for them. Oftentimes, they ventured into the basement for a closer look, but Micioni always said "the Holy Grail" was in the attic.

Shortly after his death, the family spent days cataloging Micioni's vast collection. Through that, they identified key reference points from his life. They found pins that captured his support for Robinson, the sport's first African American player. They learned that Micioni attended a charity game in 1942 that saw Ruth come out of retirement and homer off Walter Johnson. They sifted through old newspaper clippings and noticed that one of them was autographed by Mel Ott. They uncovered a letter written to Joe DiMaggio. And they found the original envelopes that contained the autographed cards that are now worth a fortune.

"He did not collect this thinking someday it would be worth a lot of money," one of his nephews, Peter Micioni, said. "His intent clearly was to do this collection and retain it for the game of baseball. He 100 percent knew that he had stuff that nobody else has -- and that his legacy really is giving this stuff back to the game of baseball."