A peek inside bond guru Bill Gross’ multimillion-dollar stamp collection that he's selling

  • Bond guru Bill Gross is exiting one of his long-standing investments and is giving all the proceeds to charity.
  • That investment is rare U.S. stamps, which Gross has been collecting since 1993.
  • "It's better to give other collectors a chance to own some of these stamps and to use the proceeds for philanthropy," Gross says.

Bond guru Bill Gross told CNBC he is exiting one of his long-standing investments and is giving all the proceeds to charity.

That investment is rare U.S. stamps, which Gross has been collecting since 1993.

"There are limits at some point to a collector's portfolio because if pursued long enough and admittedly with enough money it's possible to fill in all the spaces in an album – in other words to get them all," Gross said in an interview with "Power Lunch."

"I figured it's better to give other collectors a chance to own some of these stamps and to use the proceeds for philanthropy."

The largest known unused block of 10-cent 1847 first-issue stamps estimated to fetch between $500,000 and $750,000 at auction.
Adam Jeffery | CNBC
The largest known unused block of 10-cent 1847 first-issue stamps estimated to fetch between $500,000 and $750,000 at auction.

Donating proceeds is something he's done before. "Perhaps $30 to $40 million already," said Gross, who co-founded Pimco and now manages the Janus Henderson Global Unconstrained Bond Fund.

He plans to sell his collection of about 3,000 stamps through a series of auctions, with the first one expected this fall.

Among the treasures Gross has is a letter from the Pony Express.

The historic Pony Express cover signed by co-founder William H. Russell estimated to fetch between $75,000 and $100,000 at auction.
Adam Jeffery | CNBC
The historic Pony Express cover signed by co-founder William H. Russell estimated to fetch between $75,000 and $100,000 at auction.

He also has a block of six of the first U.S. stamps, which were issued in 1847, worth between $500,000 and $750,000.

Charles Shreve, director for Robert A. Siegel Auction Galleries and Gross' stamp advisor for the past 25 years, told CNBC that block of stamps somehow ended up in a Bible, where they remained for "dozens and dozens of years." It remains the only intact block of six of the first U.S. stamps, he said.

Also included in the collection is a set of upside-down steamship stamps, the largest known intact multiple of any inverted stamp error, said Shreve. "Stamp collectors love errors," he said.

The largest known multiple of any inverted center postage stamp error expected to fetch between $300,000-$400,000 at auction.
Adam Jeffery | CNBC
The largest known multiple of any inverted center postage stamp error expected to fetch between $300,000-$400,000 at auction.

While the auction is "at the high-end level," Shreve said he expects a good turnout.

"There will be hundreds of people around the world who will be bidding. There will be a live auction but people will also be able to bid live on the internet," he said.

The first set of stamps hitting the auction block are worth an estimated $10 million, while the entire collection is worth tens of millions.

Gross currently holds the record for the biggest single-day stamp auction in 2007 when he auctioned off his collection of British stamps for $9.1 million.

— CNBC's Kevin Flynn contributed to this report.