power of attorney?
What does HPD require from you when you say you bought another private seller's gun? Not even a bill of sale. Just the specs on the gun and the seller's name. If it doesn't come back as reported stolen, do they even bother to call the owner? All the owner has to do is sign the permit. I'm sure the HPD Firearms officers are adept at spotting forged signatures on onion skin carbon copies.
The only additional factor here is that HPD has custody. For them to release the guns to a new owner, I would think a bill of sale, possibly notarized, would suffice.
I remember reading once that a court order was needed, but I think that was in the specific instance of the person needing to expunge a felony from their record.
Power of Attorney would be overkill. It would allow the "buyer" to act on the owner's behalf in all aspects, including signing papers for him. That latitude would allow him to sell the gun to another party without the owner's consent. I don't think I'd be comfortable giving POA to anyone not a close relative.