Who has the bass actually been stolen from?
The only actual facts are that it didn't reach the buyer but was sent to Auction (possibly in error) by PF.
In which case, the auction house would have contacted PF, said that someone had been in touch and pointed out the error. PF would then have to have the bass back to correct the error. Remember several people have now contacted the auction house to alert them.
The auction house would have no way of verifying the actual owner of the bass or whether it was actually stolen.
If the police had arrived at the Auction house then the same process would be followed.
The theft is only alleged, and the identity of the person guilty of the alleged theft is unknown.
The sender of the bass remains the property of the owner until the bass arrives with the buyer, and is supposed to be in the care of PF. Let them sort it out.