>You can actually buy used solar panels at a significant discount.

The problem with used panels is that used panels don't qualify for the 30% federal tax credit because the tax credit only applies to the "original installation" (26 USC Section 25D(e)(8)(A)) and the associated labor, piping and wires (26 USC Section 25D(e)(1)). After not being able to claim a 30% tax credit on the labor, piping, and wiring, the used panels might not actually result in a lower total cost.