A Philadelphia-based comic collector who spent almost 40 years building his museum-worthy collection of underground comic art — much of it purchased directly from the artists themselves — is putting it up for auction this week.

Eric Sack, author of The Life and Times of R. Crumb: Comments From Contemporaries, will see the majority of his collection of ’60s and ’70s comic art auctioned off on Nov. 17 and 18 at Heritage Auctions. Sack began collecting in 1978, and over the decades amassed over 1,300 pages of original works from artists such as Robert Crumb, S. Clay Wilson and Spain Rodriguez. The collection achieved such prominence that it was loaned to various exhibitions later in his collecting career.

Speaking on the phone last week, Sack, 62, says he put his collection up for auction because he felt he couldn’t collect any more artwork. He had bought everything that he wanted, and could afford.

“There was nowhere to go with my collection but down,” Sack says. “There wasn’t much else for me to buy, which is a little weird to say. But short of going into collections where I had to offer stupid money that I couldn’t really afford to do, I did a pretty good job.”

Except for about 50 pieces he’ll keep from his 1,300 pages of comic art, he’s selling it all.

The most valuable pieces in Sack’s collection come from R. Crumb, who Sack cited as his favorite artist, and one of the first artists whose works he bought. Some of the pieces from Crumb in Sack’s collection are estimated to be worth tens of thousands, including a two-page original story artwork titled “Freak Out Funnies,” which was estimated to be worth $40,000, according to Artlyst.

This is a great time for Sack to sell, as the market for underground comic art has grown in recent years, says Patrick Rosenkranz, author of Rebel Visions: The Underground Comix Revolution. The global popularity of Crumb and Juxtapoz founder Robert Williams — and the success of museum exhibits featuring works from collections like Sack’s — have pushed prices for underground comix to fine art prices. For example, Rosenkranz says copies of Zap Comix #1, which were drawn completely by Crumb and sold by his wife in the Haight-Ashbury back in 1968, can fetch as much as $10,000 if they are first editions with “Printed by Charles Plymell” on the back cover.

Rosenkranz feels other collectors will start selling their collections as well, as the market appears to peak.

“Any collectible goes out of style after the people who experienced it in their own lives die out,” Rosenkranz says. “Who cares about Hopalong Cassidy anymore? But 20 years ago a Hopalong Cassidy collection could fetch a decent price.”

Still, the Sack auction marks the end of an era in the Underground Comix world. Well-known in the scene for collecting pieces in the ’80s, when the artists were practically forgotten, Sack picked up a lot coveted works for cheap. He also took his pursuit of original works much more seriously than your average collector; for example, he says he spent 20 years pushing Wilson to sell one of his earliest pieces.

“I don’t know if he told you any stories about ‘figurative’ briefcases full of cash,” Rosenkranz says, “but he’d go up to someone’s studio or store, show the cash, say, ‘I want this, this and this’ and walk out with all of it.”

Sack says he’ll miss his ritual of poring over his new purchases, examining each drawing for signs of the artist’s creative process, such as streaks of white out and doodles on the back. But his passion never rubbed off on his children, which Rosenkranz says is a common reason for Underground Comix collectors to sell.

“A lot of collectors are getting old and their kids don’t want the stuff — either they’re not interested, or they’re embarrassed by it, or they think it’s too dirty,” Rosenkranz says.

In Sack’s case, loaning out his collection to various museums led to family trips to Europe, yet his kids never showed interest — that is, until after Sack set up the auction.

“My youngest, who is in her 20s, recently posted the Artlyst article on her Facebook page,” Sack says. “In the heading she put, ‘Oh my God, my dad’s weird hobby is now the Eric Sack Collection.'”

Watch a video of Sack discussing his collection: