This answer is to be seen as supplementary information to TRomano's excellent answer.
The thing to emphasise is that, in your situation, the use of commas is usually one of style, not grammar. The only situation like this where you would be forced into your choice of comma placement is when their inclusion or exclusion changes the meaning.
In TRomano's examples the meaning doesn't change with the inclusion/exclusion of commas, so you are free to choose (although the convention is as TRomano states; if in doubt, follow it). However, consider the following sentences:
He was clubbing baby seals. (he was beating baby seals with a club)
He was clubbing, baby seals. (baby seals are being told that he was in a nightclub)
Here, the meaning completely changes with the inclusion of a comma.
Another point about your two examples is that both "selling houses" and "walking through the park" could either be considered necessary parts of the clause (conventionally without a comma), or supplementary information (conventionally with a comma).
Taking the first example, if the intended meaning is she makes good money regardless of how, you might be more likely to add a comma as "selling houses" is additional information. However, if the intended meaning is that selling houses makes good money (for her), then "selling houses" is an intrinsic part of the information so you would be less likely to include a comma. However, as this difference in meaning is so subtle as to be non-existent, the choice to exclude a comma doesn't really make any difference.
Finally, in your first example, she is the subject of both parts of the sentence.