So, the point is, is it possible to teach English through English to non-native speakers?
It depends on both the teacher (his/her ability to teach) and the non-native speaker (his/her ability to learn), but I would say it is perfectly possible in most cases, providing both parties are determined.
However, I highly doubt that the number of users on this forum who would be willing to pay money to improve their English (which would not be an easy task in a short duration) would be higher than, say, 5-10. Most of them are here just to pump out posts for the money from signature campaigns and couldn't care less.