being a mechanic and an avid craigslister, I see potential to make stupid profit on something like this...if I lived in an area where this is needed.
get 3 $500 craigslist specials (need trans/need motor/been sitting) fix them, and use this service to pimp them out...when they die, so what. offer to sell them to whoever rents them.
they bring customers to me, and take care of the liability.
that being said, I'd never let randoms drive my driver/projects. someone would do something stupid, and who's gunna buy me a new motor?