Trading the Dhamma is wrong, according to Ud 6.2.
But selling a paperback book containing words of the Dhamma is not wrong, if the proceeds meant for the author, goes to charitable causes.
Ven. Ajahn Brahm has published books and has regular talks on YouTube. According to this page:
Ajahn Brahm has authored three popular books that are available
Would Dhamma made avaliable by commerce take one out of this wheel?
Making a bussiness out if Dhamma is a corruption in one who does it, it doesn't corrupt the Dhamma.
Therefore there is no apparent reason to assume that one who payed to learn the Dhamma would be deficient.