Getting expert advice for what you're struggling with is amazing, but what can you say or do to make the talk not only beneficial to your side? How can you repay the favor when you're not the expert?
You can always share the fact that you found someone's advice helpful. Online, you can say something like this:
"If you're ever in need of [blank] advice, talk to [so and so]. They just helped me out with a question I had."
Where? If you have a blog, maybe add a random P.S. If there's a place to leave a review, consider that. Or if you're on social media, maybe share a thumb's up with people. They may not need any advice today, but maybe in a month or a year somebody will. Or maybe a stranger, searching online, will stumble across your thumb's up.