I've watched my neighbor's dogs many times and they've watched mine a few times. I don't think I would use a service like this. I knew and trusted my neighbors quite a while beforehand. Meeting people over a web site, then entrusting my dogs lives to their care isn't something I would want to do.
I'd be more willing to take care of other people's dogs, but don't know why I'd volunteer to take on the work. Even in exchange for money I probably wouldn't do it.
That said, I do think there is a market for this. Plenty of people are not nearly as cautious (paranoid) about their dogs as I am. There are also probably a good number of people that would do it for small sums of money ($20-$30/day). The dog kennel industry seems way too expensive to me. It shouldn't be so expensive to take care of a dog.
Cracking this kind of market is hard though. It's mostly a marketing/brand job and if that's not a strong suit of yours I'd stay clear away.
Actually I'd much rather you start a dog kennel that is cheap as possible while maintaining safety and quality. A simple safe place to drop off my dogs while I go on vacation. I don't see why that service can't be offered for $30/day or something.
I don't think anyone has managed to build a chain of well respected and affordable dog kennels in the US.