One old adage that's certainly true is "One man's trash is another man's treasure." One person sees an ugly hunk of metal, another sees a sculptural work of art. One woman's Pollock is another whose Kid Could Paint That.
Most collectors are well aware what the ultimate prize of their type of collection would be. But people who aren't familiar with collectors' items may be completely oblivious to the value of such items. That's why flea markets, garage sales and the like will always have their treasure hunters.
The article linked below lists what the author estimates to be the most valuable items for ten different types of collections. For example, the book in the photo above is the most valuable printed book, the Bay Psalm Book. Said to be the first book printed in English in North America, this volume is the interpretation of the original Hebrew script of Psalms by the Puritans. Publication began in 1640, with approximately 1,700 copies printed. Eleven copies are thought to be currently in existence.
Old South Church in Boston once owned two copies of the Bay Psalm Book, one of which they sold in order to pay for ministries and repairs. The book sold for $14,165,000 on November 26, 2013 to David M. Rubenstein of the Carlyle Group.
Read about nine other most valuable collectibles of their types here.