Welcome to SCN!
I'm new into HANA, but I will try to help you with lot of information that I collected...
- What exactly I need to develop and sell HANA applications?
You can use the SAP HANA on cloud, it's the popular method there...
There's some types of deployment options, so you need to choose one that will best fit in your goal. If you are just practing you don't need SAP Hana One for sample...
http://scn.sap.com/docs/DOC-31722
- What are the sales methods?
This is all about you and your customer. But you can't sell a developer edition enviroment. You need to provide a production enviroment to your customer (like SAP Hana One).
- Who can buy these applications?
Everybody. SAP Hana is a platform that you can develop any kind of app. You can develop mobile app tool for free as you can build a robust system for bigger companies.
- If I develop a cloud application, the buyer need to have a HANA system of his own to use it or his data will always be stored in my cloud HANA database?
Your customer will need a Hana instance. He can afford Hana Appliance on his own hardware setup or rent a cloud service and choose the Hana deployment type. So you can sell your application through delivery units.
Check the deployment types of SAP Hana below:
http://www.saphana.com/community/learn/deployment-options
Best Regards,