Sometimes it’s sheer serendipity. There’s no shortage of stories where people discovered highly valuable collectibles, including dated currency bills, in attics, books, dressers and photo frames of old or deceased family members.
How to care for them
Regardless of their age or rarity, banknotes that are damaged — torn and tatty, or frayed and faded — aren’t worth the paper they are printed on. “They need to be stored in special holders made by companies like Lindner or Lighthouse and protect them from humidity and human handling,” Razack said.
These plastic holders that cost less than $50 “protect the note from fingerprints, folding, and stains,” said Weinberg. “Once placed in the holder, you shouldn’t handle the note too often.” Banknotes that are damaged — torn and tatty, or frayed and faded — aren’t worth the paper they are printed on.
Handling a banknote too much can transfer the dirt and oil from your hands onto the note and can cause folds and creases that trap gunk.
Stapleton cautioned against the classic rookie mistake: treating and tampering with a banknote. “Don’t wash or press a banknote, or try to erase any marks on the note, or trim the edges off,” he said noting doing so takes away the note’s original sheen, which is detrimental to both its grade and value.
The bottom line
Although it’s a passion project, Millensted said “serious collectors want to make sure they have bought wisely and that there is a hope of a profit when the time comes to sell.”
The good news, said Stapleton, is that “over the last quarter century, your average return on investment has been ten times.”
And while Razack said his collection would never be up for sale, he admitted: “Over time this has the potential for value growth, which is far greater than any other asset.”
To comment on this story or anything else you have seen on BBC Capital, please head over to our Facebook page or message us on Twitter.