We have been developing courses for clients and recently started selling our own, but it is difficult to get online purchases. Any tips?
It depends on the type of customer you're targeting.
If your courses are for professionals, you could try something like Paid LinkedIn Advertising, targeting people in jobs who could benefit from your courses.
If your courses are targeting students, you could try Paid Facebook Advertising, targeting people in interest categories related to the courses.
Search engines are highly competitive battlegrounds for eLearning and your user experience, value proposition, credentials and pricing all need to be excellent to win new business there.
Pre-roll video would be a good strategy - YouTube ads targeting content/videos related to your courses. This is a higher cost strategy however, because you'll need to produce the videos.
Wherever you advertise remember that learning is aspirational (emotional) - so I'd try to use messaging and calls to action which are anchored in emotion, as opposed to pushing pricing or basic benefits.