Seems like a really good idea to me but i think you'll encounter more problems than it's worth. i.e

1) Gettin around health and safety issues, insurance etc... just incase somebody injures themselves and then looks to claim off you!
2) Guy takes his car apart then the part he has for the repair is incorrect. he won't be able to get correct part till the monday morn and it's a car of a decent value and the owner doesn't want it leaving outside over the weekend. Will you then have room to park the vehicle?

I'm sure others will add to this but i do think it's a good idea in principle.