When buying individual bonds, stick with new issues when possible. You get them wholesale. Older bonds are more dicey. They trade on the secondary market, and their prices include a dealer's markup. Don't invest all your retirement money in bonds.
Inflation erodes the value of bonds' fixed interest payments.
Stock returns, by contrast, tend to keep pace with inflation. Young and middle-aged people should put a large chunk of their money in stocks. Even retirees should own some stocks, given that people are living longer than they used to. Consider tax-free bonds. Tax-exempt municipal bonds yield less than taxable bonds, but they can still be the better choice for taxable accounts.