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.