The Virginia woman eyed the box of kitsch at the West Virginia flea market and figured she’d discovered a true steal. Surely, she calculated, she could resell that brown leather Paul Bunyan doll to folk art enthusiasts for a tiny profit. As for the rest of the box’s items, she loved the plastic cow. The cow would get displayed in her living room.

But the third item, a painting — with swirls of green and pink, carrying a plaque emblazoned with the word RENOIR — did not excite her so much. She liked only its golden frame and assumed the thing was a fake.

So the woman forked over $7. She drove home. She stuck the box in a shed.

Now, about two years after her random trip to a flea market on Route 340, the woman is selling the painting through the Alexandria-based Potomack Co. auction house, which determined that the piece is a bona fide work by Pierre-Auguste Renoir, the renowned French impressionist. Potomack thinks the painting could fetch as much as $100,000, if not more, when it goes on the auction block Sept. 29....