i used to own a towing business you will want too make sure you have the right insurence once to put a car on the trailer ..ie when i backed my truck up to a car and put the cable to whinch it up i was covered by what was called hook insurence .
so if the cable broke the car owner was covered . once all 4 wheels are the tray of the truck it was called load insurence both come under a seprate policey and 2 lot's of premuim's .
you may want to check this out b4 offering the service . also seening how your effictly towing a customers car you may need a toweys lincence .
this is the main reason workshops find it cheaper just to ring a tow truck . i would not want a car of mine on your trailer not knowing what is covered and by who if you prang or the car falls off on the way to the workshop .
not trying to put you guys off its hard to make a buck in these days and times and good luck with the workshop just making you aware of what you might be up for .
cheers dean