I see its opensource, is it completely free, I can host it on servers and sell it?
Please check and read the whole post.
It depends. ERPNext is 100% Free and Open Source and depending on your scale and capability, self-hosting may be the right solution for you. Learn more about our enterprise services.
If you are a company, the cloud may make more sense for you. If you setup ERPNext on your own, then you start with a base cost of around $40/month for a VPS, then you’ll need to pay extra for email, security and backups services. After that you’ll spend a few hours installing and setting up everything, and also set aside a few hours of your time each month for maintenance + software updates, or maybe get someone to do that for you. Which gets expensive pretty quick. Frappe Cloud on the other hand, takes away all of that so you can focus on what you do best.