The Declaration of Independence
Somewhere out there in the world (presumably mostly in the U.S.) there were once 500 copies of the original printing of the Declaration of Independence. Only 23 copies have ever been found, and one turned up in a flea market in Adamstown, Pennsylvania. A financial analyst purchased an unassuming oil painting for $4, only later to learn that an original copy of the Declaration of Independence was hiding inside. When this was sold at auction in 1991, and went for an eye-watering $2.42 million.