Even if you close your Amazon AWS® account: your EC2 reserved instances will continue to be billed (that is nice!).
However you can register as a seller on the Amazon AWS® marketplace to sell your EC2 reserved instances. There is a requirement for this: to enter a US bank account. This can be achievable: especially if you want to sell no upfront instances, you will have no money to collect thus no money sent to that US bank account (a friend, affiliate, or gift card US bank account will do).
Then here comes other stupid rules from Amazon AWS®, get ready for it:
Only Amazon EC2 Standard Reserved Instances can be sold in the Reserved Instance Marketplace. Amazon EC2 Convertible Reserved Instances cannot be sold. Reserved Instances for other AWS services, such as Amazon RDS and Amazon ElastiCache, cannot be sold.Sell in the Reserved Instance Marketplace – Amazon エラスティック コンピューティング クラウド
So if you have any Convertible EC2 reserved instance (non-Standard) you are stuck with them until the expiration of those instances.
Thank you Amazon AWS®!