Can I still sell an AWS Reserved Instance if only paying for it monthly
On the AWS RI Marketplaces you can't. They clearly state in the conditions that AWS must have received payment upfront.
"Reserved Instances can be sold after they have been active for at least 30 days and once AWS has received the upfront payment (if applicable)."
https://aws.amazon.com/ec2/purchasing-options/reserved-instances/marketplace/