Amazon has set up a plce where you can buy and sell reserved instances. This helps in getting rid of reserved instances you may no longer need.
I am more interested in buying a partial reserved instance than selling them. So I will not be going over selling, if you want to sell a reserved instance this video by AWS will show you how to
Amazon charges 12% of the selling price, you can only sell your remaining months left.
Amazon has a video for this as well.
But, here is my example
Click on Sign up Now
You may need to log in with your username/password
Now you will get a confirmation email
Open the EC2 console
Click on Reserved Instances, and click on Purchase Reserved Instances.
This search screen comes up
I am only interested in buying short term ec2 instances from third party sellers.
Instance Type: t1.micro
Availability Zone: Any
Then I selected
Term: 1 month – 6 Months
Offering Type: Heavy
Click on Search
Two different third party sellers show up. Both have the same term 4 months. One is cheaper at $17.00, the hourly rate is always what amazon specified. One is in the us-east-1d zone and the other in the 1b.
Looking at the normal 1 year price of $62 up front (as of 12/21/2012)
This would break down to $5.16 a month. For 4 months that would be $20.67. So the $17.00 is a good deal over what amazon directly charges. The $20.64 is almost exactly what amazon charges but with a shorter term commitment.
Looking at the other site one has a 7 available and the other 8.
Enter the desired amount you want and add it to your cart.
Then click View Cart
Now click Purchase.
That is it
For those of you like me who like using the command line tools. There is a way to check availability of reserved instances
http://docs.amazonwebservices.com/AWSEC2/latest/CommandLineReference/ApiReference-cmd-DescribeReservedInstancesOfferings.html  Using the ec2-describe-reserved-instances-offerings command
Here is an example
> ec2-describe-reserved-instances-offerings -t t1.micro --offering-type "Heavy Utilization" -z us-east-1a -d "Linux/UNIX (Amazon VPC)" --max-duration 6 --headers
This will only look for
Only Linux for a VPC types
With a maximum duration of 6 months
> ec2-describe-reserved-instances-offerings -t t1.micro --offering-type "Heavy Utilization" -z us-east-1a -d "Linux/UNIX" --max-duration 6 --headers
This is the same command but searches for non VPC instances
Here are the results
It may take a little effort but its a nice way to save some money
 Amazon Reserved Instance marketplace
 Selling in the Amazon EC2 Reserved Instance Marketplace