CÚCUTA, Colombia—For years, the hallmark of this town on the border of Venezuela has been the pack-laden currency traders offering desperate folks from across the river something of value in exchange for their inflation-racked Venezuelan bolivars.

Now many of those traders are on the losing end, stuck with more than a billion worthless 100-bolivar bank notes since Venezuelan President Nicolás Maduro this week voided his nation’s largest bill and temporarily closed the border with Colombia.

“Maybe...