I've only ever worked at clinics that carried them on consignment. We would distribute them to a patient if ordered by a physician and then the device vendor would handle getting reimbursed by the third-part payor. In many instances, the PT could charge for a patient education code which was reimbursed poorly, but it was better than $0.00.
I've no idea how much one could receive if they purchased the TENS units in bulk, dispersed them, and billed insurance for it.