Curiosity got the better of Christine Ridout a year ago when she browsed through an estate sale at a Buffalo home and spotted the gray cardboard glove box on a kitchen table.

Inside were six fragile, tattered documents. The timeworn parchment and graceful handwriting were eye-catching.

"I asked, 'How much?' and was told three dollars, so I said, 'OK, I'll take them,'" said Ridout, 52, a veteran bargain-hunter who hit more than dozen yard sales that day. "I had no sense of their history or value."...