Hi Rickh2677,
1. Do you have any evidence to prove that the PA system sold is yours? (e.g. receipts, bar code number, photographs and unique markings on the system)
2. Write a letter to the buyer informing them that the PA system is your property and you did not give the seller power to sell the system. Request the system be returned to you. The buyer will then have an action against the seller for return of the purchase money (action in misrepresentation or breach of contract)
3. If the buyer refuses to return the goods, you have an action in tort (detinue), which is a private property offence, against the buyer. This essentially means that the buyer is in possession of goods (the PA system) which belong to you (i.e. you have better claim to the goods, being the true owner) and they refuse to redeliver the goods. If this fails, or concurrently, you also have an action in tort (conversion), another private property offence, against the seller (the friend to whom you loaned the goods), for selling something that belonged to you without your consent. If successful, you will receive either the PA system back (from the buyer) or payment of the value of the PA system (from the seller). To be successful, you need to have evidence showing that (i) you are the true owner of the PA system; and (ii) someone sold your PA system without your consent; and (iii) that someone was the seller, or the buyer now has your PA system.
Depending on the cost of the PA system, the action will likely be heard in the small claims court in NSW.