>> holy smokes. >> reporter: in her possession, this collection of professional baseball card passed down through five generations. cards so rare, experts didn't even believe they still existed. >> this archive was by far the most exciting that i had ever seen. >> reporter: the owner's great, great grandmother ran a boardinghouse in boston in the 1870s back when the boston red sox were known as the red stockings, and she housed the players. the grand slam, a letter to the landlady with notes from three hall of famers about how they missed her cooking. those card now worth at least $1 million. >> reporter: not the first time regular folks have stumbled on a hidden treasure. like the man who bought this painting covering a hole in his wall for $30. and later sold it for $1.25 million. or this, one of the original copies of the declaration of independence, bought at a thrift shop for $2.48. worth almost $500,000. new reason to search the attic and study that heirloom.