One of the potential risks of buying reserved instances on the Amazon cloud is that you might buy more than you need. This has been a consideration for me for hosting the Keyapt app. Now Amazon lets you sell excess capacity to other users.
Amazon EC2 Reserved Instance Marketplace
The Reserved Instance Marketplace gives you the flexibility to sell the remainder of your Reserved Instances as your needs change, such as wanting to move instances to a new AWS Region, changing to a new instance type, or selling capacity for projects that end before the term expires. Once listed, other AWS customers can view the available Reserved Instances from the AWS Management Console, and choose the instance that best matches their criteria. AWS will complete the transaction, transferring ownership of the Reserved Instance to the buyer. Sellers will receive a cash disbursement for their Reserved Instance directly into their bank account via wire transfer.