You are thinking of starting the equivalent of a private practice as moonlighting?
I'm not certain about this, but I think that might be illegal. From what I understand, moonlighting has to be approved by the program before you can do it, and you have to be working in an institution, not on your own.
If I were you, double check the legal fine print before you do this. Your program has to approve of what you're doing before you do it anyways so they're a good start. Do not go over the program's head and intentionally leave them in the dark. That is illegal. I've seen residents do this, and those that have got off with a slap on the hand because IMHO the PD was too wimpy. If a resident did that in my program, I'd seriously consider kicking that person out with strong words on their record that they at the least did not do their responsibilty to make sure they were engaging in a legally accepted practice.
I can tell you this, even if this were legal, if a resident were to want cash-only and I was the program director, I wouldn't let it happen. IMHO the resident in this case is too unrestricted. There is literally no one the resident has to answer to, and if that resident did something that went wrong, you betcha a lawyer will be thinking of pointing the legal guns and trying to find the program liable.
Another problem with starting cash-only at your point in your career is there are several things you have to consider if you want out of this practice. You will likely want to move onto bigger and better things by the time you graduate. You will be required to refer your patients to another doctor, and depending on your state may be required to give 3 referrals, typed up, with specific office appointments made for the patient and mailed to them via certified mail.
That's just the tip of the iceberg. Are you certain you know what you're getting into? I didn't mention you may be required to keep your patient's records in fire-proof cabinets, hold onto the records for at least 3 years, have several forms written up for the patients such as release of information forms, waiver forms, consent forms, etc.
Do you really want to have to buy a weight scale, BP monitor, fire-proof cabinets, type up a bunch of forms, just for a moonlighting gig?
(Are you manic?)