while that sounds like a decent idea to offset the initial cost of the compressor, since most folks who own them for "personal use" don't have any liability insurance for 'selling air fills', you're easily opening yourself up to possible lawsuits in the rare case that a diver had an accident and died after getting an airfill from your compressor, even if the death is unrelated to the air fill.
you get a family member, or spouse that has lot a loved one, their lawyer will include anyone they can in the lawsuit. THe place he bought his gear, the instructor that taught him to dive, the dive boat owner, the captain, the mate, the maufacturer of his regulator, the last person who worked on it at the LDS, and the tooth fairy.
At least if you're going to do it get them to sign a standard time liability release form. If they won't do it, then screw them... don't give them an airfill.
I've seen this happen in general aviation. a drunk breaks into an airport, steals a plane and takes off and hits the ground at over 100mph, kiling him. His family sued the airport, the mechanic who worked on the plane, the aircraft manufacturer, the aircraft owner he stole it from, and the manufacturer of the seat in the airplane. They sued the seat manufacturer because the seat broke loose in the 100mph + crash as the airplane crumpled around him because they said the seat should haven't come loose. Now remember this guy was drunk and stole an airplane and wasn't even a pilot.
