The 144 cards -- wrapped in napkins and in their original envelopes -- included a complete set of 1914 Cracker Jack cards of legendary major-leaguers Ty Cobb, Christy Mathewson, Honus Wagner, "Shoeless" Joe Jackson and others.
The sale, believed to be a record for baseball cards, is expected to be announced Monday by Willowbrook auction house MastroNet Inc.
"No one could predict it would sell for this number," said MastroNet President Doug Allen. "We're in a business where people call us all the time and tell us 'Hey, look at what I have here' and it's rarely a gold mine. This one was."
Both buyer and seller have asked for anonymity.
The cards belonged to the woman's uncle, who died during World War II, and her father kept them until his death. After some research on the Internet, the woman realized the cards were valuable, Allen said.
One collector said he was in a "state of mini-shock" when he saw the cards.