Hi Fong,
You can see the Sales History in two different places in Adagio. In Inventory, the Transaction History tab in the item inquiry allows instant lookup of an item's transaction by date range and transaction type:
When entering an order, you can instantly lookup the Sales History and pricing of an item to a specific customer by clicking on the "Prices" button and opening the History panel:
Perhaps someone could show the sales staff these buttons? ;)