Currently involved in car sale?
If you have his permission, then just sell it as if you are him.
Sign his name and give the buyer a bill of sale.
(assuming its ok with him and you give him the money, you have not defrauded anyone)
Legally, all you need is a power of attorney authorzing you to sell it. I think a power of attorney needs to be notarized and therefore a fax might not suffice, you might need the original.
Some buyers might question you without a power of attorney. You might be able to calm their fears by giving them the VIN # and telling them to call the police to make sure its not stolen.