I recently found out we are allowed to use TD's API if we have an account with them. However I am not entirely sure if we're allowed to use it to create our own applications that we want to sell on the market. I went to their site: https://developer.tdameritrade.com/content/getting-started and it says Each user is allowed to create only one app for private, non-commercial use. To inquire about commercial usage, please Contact Us. I sent them an email but I havent recieved a reply from them. I tried calling and the guy at first said we're allowed to but he sounded uncertain and then told me to email these guys instead. I would think if we're trying to use their API for making money then they would probably want a commission fee or something.