I’m not sure, but I was under the strong impression that freelancing was prohibited for foreign workers in Taiwan under most circumstances.
Reasoning: In order to do any work legally, you need a work permit. In order to get a work permit, you need an employer. You cannot be your own employer here (self-employed) in most circumstances. The only exception to that restriction that I am aware of is getting the permanent resident ARC (APRC?). Marriage or staying here long enough can qualify you. If you do that, then the working restrictions are lifted.
If this is accurate, the OP should indeed beware of posting advertisements for privates. I have actually heard that example given as one of the main reasons for deportation for illegal working (i.e., someone who drew unnecessary attention to his/her unofficial teaching).
Maybe someone who has first-hand knowledge/experience will confirm or clarify this.