I think at that point you wouldn't be "crowdsourcing" your labor anymore. It'd be like a telecommunications relay service[1], where the operators are properly trained employees.
What if there was a setting: "I only provide service for free" / "I accept payments for my service" in your profile?
Such a setting could even be on a per-case basis. Imagine if you helped someone for a good part of a day, and they were ready to return the favor in cash, and you were given the choice to accept or decline it.
Similarly, I wouldn't help an old lady cross the street then ask for a pound.