What's the best approach to sell agile / scrum coaching and training services to companies?

You can do this through having a webinar for corporate executives. Run traffic to the webinar through Facebook and LinkedIn then at the end of the webinar have a soft sales pitch. Follow up with all participants through email and messenger.

The one thing you must accept is that each and every company has their own style meaning it might be flavors of agile/scrum, kanban, waterfall. Before you sell understand how they operate and it may be working for them even though you think otherwise.

Ultimately, it's what the engineering team is most comfortable with and their level of experience

If you want to chat. Happy to dig in deeper.

I 've done project management consulting for years, using Scrum , XP , as well as the classic PMI approach, and a blend of waterfall and spiral.

People think that Agile is a silver bullet that will solve all their problems, which is not true.

You need to do your gap analysis and due diligence , and show the company the gaps in their implementations, and the problems they have. If Agile ( and scrum) will be the right fix, you then should show them how it does it. Afterwards, you do basic high level training, coaching, sharing ideas, , then take one team / project as an example,. and do the thorough change management by co-managing the project with their managers ( or scrum master and product owner if they have the right structure) . They will be shadowing you in the beginning for one or two sprints, then you drive and they lead after wards and you keep advising them until things are stable .. But just giving training or theoretical lecture is not supposed to deliver a real result.