According to AARP, this is a common scam that has cost grandparents millions of dollars. Unfortunately, the tax rules don’t help in any way. The deduction for a theft loss—assuming such scam is a theft under state law—cannot be claimed in 2018 through 2025.
The difference between face value of a bond and lower market price, attributable to rising interest rates. On a sale, gain on the bond is generally taxed as ordinary income to the extent of the discount.