"their scam was selling fake chinese medicine to old people." and "their scam was to sell fake chinese medicine to old people."
I'm not a teacher,but I think that the first lasted longer. They started selling medicine and did it for a while for a living (for example). And the other one means that they maybe accidentally had some fake chinese medicine and decided to sell it. So,they would sell that bottle or two and that would be it.
Student or Learner