I'm not sure this is a scam.
It may just be a potential buyer that is afraid of sending large sums to someone sight unseen before the item is delivered in the stated condition.
don't get me wrong. I wouldn't send a copy of my passport anyway.
If I am correct, there are escrow services that can be used that are legitimate and will keep the funds the buyer submits in escrow until the package is delivered.
I would contact the potential buyer and suggest either paypal or an escrow service. If the potential buyer isn't happy with those suggestions, I would walk quickly from the deal.
I don't like paypal because it protects the buyer and not the seller at all, but options are limited.
I prefer that buyers agree to a purchase in writing and them come pick up the unit personally. That way, not only can I connect the unit to let them hear it (after they have agreed to the purchase. I'm not a stereo store one can demo equipment), they also would know for certain it works and looks as described.
Another suggestion is that they empower an agent or friend to come and pay for the equipment and take delivery.
enjoy