The 5 highest valued transactions are related to sales, regardless of your type of customers. It should be based on the invoices/receipts (inclusive of GST) issued to customers.
Where the semi-annual return form only allows you to select one product, please select the product accounting for the highest value amongst the products in that one transaction, if the invoice/receipt consists of multiple PSPM products. For example, if the total sales stated in the invoice is $25,000 which consists of 1 loose diamond ($8,000), 1 loose sapphire ($7,000) and 1 gold bar ($10,000), you can select "Gold Bullion (e.g. pure gold in the form of bars, ingots, or plates)" as the PSPM product and indicate the transaction value as $25,000.