You can walk out of the Savers in Manchester, N.H., with a shirt for as little as $4.99. There are $2 rings for those interested in costume jewelry. Framed artworks too big for shelves lean invitingly against one of the walls.

The thrift store’s manager says that shopping there is like a personal treasure hunt, which was certainly the case for one woman who made the find of a lifetime in 2017.

While pushing a metal cart through the Savers on a hunt for home décor, she spotted a dusty painting with a substantial wooden frame amid a stack of posters and prints. Two women in the image appeared to be in a standoff, the elder displaying stern disapproval.

The shopper was taken by the painting — Who were the women? Why did they seem so tense? — and clunked it into her cart. Minutes later, she was wheeling out an authentic oil panel by N.C. Wyeth, one of the premier American illustrators of the 20th century, known for bringing to life classic stories like “Treasure Island,” “Robin Hood” and “Robinson Crusoe.”

Her bill for the long-lost work, which is expected to sell at auction on Tuesday for between $150,000 and $250,000? Four dollars.

“We look at all the donations we receive and we place a value on them as best we can,” said the Savers manager, Shaun Edson. “We are not connoisseurs of paintings. We do our best to evaluate the pieces and value them appropriately.”

“I wasn’t with the company in 2017,” he added. “Obviously we missed the boat.”

The painting’s owner recalled the sequence of events in a telephone interview with The New York Times and was granted anonymity because she is about to become much richer from the sale of a painting that has drawn considerable attention.

After purchasing the piece, the buyer hung it in her bedroom before eventually casting it aside into a closet with school pictures and other items. When she rediscovered the work in May and noticed a signed label on the back, she decided to post some images of it on Facebook.

Lauren Lewis, an art conservator, was among those who reached out after seeing the painting on social media. Eventually, the owner said, she and her husband wrapped the painting in a blanket, loaded it into the back of their S.U.V. and drove about 90 minutes to meet Lewis in the parking lot of a bus terminal.

Wielding a magnifying glass, Lewis became excited and spoke about aspects of the work the owner had never considered, such as its brushstrokes.

So the couple scrapped their plans to stop for lunch, instead driving straight home to rehang the painting. This time, the owner’s husband placed pillows on the floor beneath it.

“How is this possible?” Lewis said she had been asking herself since she first saw the Wyeth on Facebook. “It’s all very fortuitous. This has all come together in a lovely way.”

The frontispiece illustration that is now being auctioned was part of a four-image set that Wyeth contributed for a 1939 edition of Helen Hunt Jackson’s novel “Ramona,” which revolves around an orphan living in Southern California after the Mexican-American War. The illustration, also called “Ramona,” portrays the tension between the young woman and her foster mother, according to catalog notes provided by the auction house Bonhams Skinner, which also say Wyeth was known for his “vivid colors and a skillful use of light and shadow.”

Wyeth’s work is featured at the Brandywine Museum in Chadds Ford, Pa., and he is part of one of the most prominent artistic families in the United States. The Museum of Modern Art owns the painting “Christina’s World” by Andrew Wyeth, one of N.C. Wyeth’s five children.

Experts say “Ramona” was probably a gift from book publishers to an editor or to the estate of the author. But exactly how it wound up at the Savers in Manchester is unclear.

Good things like the thrift store find, the painting’s owner said, do not often happen to people like her and her husband. They have begun to allow themselves to think about the bills they can pay, and a vacation to Germany to visit one of their children.

Aware that she will soon part ways with the painting, its owner bought a copy of the 1939 version of “Ramona” on Amazon. Although she is a bibliophile, she plans to rip out Wyeth’s illustration of the two women and frame it.

Alain Delaquérière contributed research.