You may be able to save on future EC2 expenses by selling an unused Reserved Instance for less than its true value or even $0.01, provided it is in the “Heavy Utilization” class.
In the description of the Heavy Utilization Reserved Instance, is this statement:
you pay […] a significantly lower hourly usage fee, and you’re charged that lower hourly rate for every hour in the Reserved Instance term you purchase [emphasis added]
What may not be clear to the casual reader is the fact that when you purchase a Heavy Utilization Reserved Instance, you commit not only to paying the one-time up front cost, but you are also committing to paying the hourly charge for every hour of every month, even if you are not running a matching instance!
The Light Utilization and Medium Utilization descriptions state: