How is the RI Marketplace different from the AWS Marketplace?

The Reserved Instance Marketplace  is, practically speaking, much like the stock exchange. The only difference is that instead of trading stocks, the RI Marketplace trades in second-hand Reserved Instances. What this means is that if you have over-provisioned any RIs, the surplus can be sold off to a buyer who needs them for the remainder of their set term.
The AWS Marketplace is a digital catalog that customers can use to find, buy, deploy and manage third-party software and services to help run their businesses or build their own solutions. It is important to note that AWS native services and solutions are not found here and, as such, financial commitments like RIs are not traded on the AWS Marketplace.