You don't need an expensive platform to sell your courses, however you do need an online platform, otherwise you are missing out. I built my own online courses and I explain in detail how this can be done without a huge investment:
https://funnelxpert.com/online-course-plugin-wordpress/

I use WooCommerce and PayPal, neither of these requires any investment. The plugin that I used for building my course costs $218 and it includes lots of additional features, such as lead generation, landing pages, etc.

Offering a small section of your course for free helps you get leads.