I don't know how else OP can make them believe in their good faith, it's difficult, I know that marketing within the forum is something that can be very profitable but you have to know how to do it, the people in the forum are not stupid at all.
If his intention are really clear on selling those poker courses, he should post all the details about it. Based on his previous post, he will only entertain members that are interested via private message which I think seems fishy. It will even help him lessen some time catering to each PMs if he post the details. I've also been wanting to learn poker recently and his post got my attention but how he acts is not appropriate if he really is a legit seller.
You don't need marketing for this, you need a well-known poker name! Who will buy a poker course from an anonymous player? He should provide his accomplishments first if there are any! Why would we believe that his lessons are better than watching some players on youtube for free?
It's easy to learn to play poker. A few tricks on how to be good at predicting what cards other people have, do you have the patience to wait for your hands, how to recognize and call some bluff, and much other stuff can be found around the internet for free. But whether you are talented or not is something you will see when you start playing! A poker course can be good, but only if comes from some professional player with years (decades) playing poker.