The ice cream man, along with a few others in my neighborhood, want me to build them a bike. They said they'll pay for the parts upfront, and then they'll pay for my labor upon receipt. I would probably charge something liek 75-125 depending on how much work is put into it (different seat, bars, forks, shift kit?).
My worry is what you guys already talked about. Taking cash does not void you of any liability, neither does not giving a receipt. Also, these engines need tinkering with. I'm worried about a jack-A$$ being stupid, messing up the engine, coming back saying I screwed it up for him.
So, like I said, many people want me to build a bike for them, I tell them I'll help you and guide you building the bike, but I won't do it for you...
If you do, make sure you have an iron clad no fault clause!!!