> Let's assume I run a survey company, and I invite a
  > bunch of people over for a BBQ. I then monitor their
  > interactions and sell that data. Did the guest pay for
  > the BBQ?

If you told them ahead of time that you would do this, then yes, they paid for the food by giving you some information about who they like to talk to at lunch. They would be customers.

If you did not tell them, then you spied on them without consent, and they would be victims.