Hi kids_mum,
First, whose playstation is it? It would not be difficult for your older son to argue it was his playstation.
Second, how old is your older son?
Third, are you sure it is with cash converters? You should contact them, then write them a formal letter, informing them the item was sold to them without the consent of the true owner and they should not sell the item until the matter has been resolved.
Forth, do you know how much your son received for the playstation? If not, you may be able to find out from Cash Converters.