In my opinion the best way to show an addon is youtube video with voiceover.
Othervice: some forum post with a few gif’s quickly shows what the addon are doing, how it solve some problems, make life easier.
As an example of good gif’s: Mesh Align Plus
Just a few sec and you can clearly realize what the addon are doing and make a decision did you need this or not.
As an example of good video: https://www.youtube.com/watch?v=WdqpxrTLnOc
If addon are complicated, i.e. it contains several functions/operators its best to make a video with a marks on a timeline to different functions to help potention customers to navigate and, probably, find some function they need.