Every character with the Bounty Hunter career starts with an Imperial Peace-Keeping Certificate (unless they opt not to for whatever reason). This is what gives bounty hunters their legitimacy. From other sources, this appears to have a one-time cost of 1,000 credits, and anyone without a serious criminal record can get one.
What I'm curious about is whether bounty hunters can subcontract agents to work directly under their IPKC. This would include hunters in training, specialists (pilots, mechanics, doctors, etc), and perhaps even backup muscle. Would it be reasonable that these types could be covered by the IPKC of a bounty hunter 'team leader' without them having to purchase their own IPKCs? The bounty hunter holding the IPKC would, of course, be responsible for any breaches of law performed by those operating under his/her IPKC.