Anecdote in support of your claim: A friend of mine who now lives in SF was recently visiting and this 'uber for dog walking/sitting' business came up. He said

Anecdote in support of your claim: A friend of mine who now lives in SF was recently visiting and this 'uber for dog walking/sitting' business came up. He said that it's very common for people (himself and his friends with dogs who live in the city) to do exactly what you state: Find someone who doesn't suck using the service and make a deal on the side with them for cheaper than what they would pay through the service.