They informed me that 1 of my orders from 3 March has been returned to eBay head Office Sydney.

That sounds suss. If the buyers correct address is on the parcel, why wasn't it delivered to them?

Was your address on the parcel as sender?

If there was no return address, I doubt AP staff would be 'guessing' where it should end up and send it to an address not on the parcel.

If a parcel of yours got lost around March 3, wouldn't the buyer have contacted you by now about it?

What phone number did you use to call Customer Support? One from the eBay website or one these strange people gave you?