Hi Eithne,
A Cost of Sales Account is only required if you Sell Inventoried stock.
If you select I Buy, but not I Stock, then you need to select an Expense Account, as each purchase is expensed to your P&L as the purchase is processed. You could use a 5 or 6 series account called Purchases or Materials etc.
If you select I Sell, then you need to select an Income account for each Item Sale.
If however you select I Stock for an Item, this changes things significantly. For example if you chose I Stock and I Buy for an Item (but not I Sell), this in effect is telling MYOB that this item is a component for a different final Item. For example if you buy bolts, and you want to inventory them but not sell them, as they are used to make a car which you will sell, then when you purchase Bolts, the only account you need is an Asset Account for the Inventory. All Purchases will be added to that Asset account, they will not be expensed to the P&L as a purchase is processed. The expensing will happen when you sell a Car.
If you Buy and Sell an Item, and you choose I Stock, then you need a Cost of Sales account. This is because when a Purchase is made, the Inventory Asset Account is debited - the purchase is not expensed to the P&L.
Say you purchased 10 Buckets for a cost of £120, the entire £120 is added to the Inventory Asset account. Then when you sell an item, MYOB calculates the average cost of all stock on hand for this Item, and expenses part of it via the Cost of Sales account. Say you sold 4 Buckets for £80. MYOB will credit Sales Income with £80 and debit the Receivables Accounts £80. It will also debit Cost of Sales with £48 and credit the Inventory Asset account with £48. This is how, if your accounts and Items are set up correctly, the purchase cost is expensed to the P&L - via the automatic Cost of Sales transaction.
Hope this explains what should be happening.
If an Item is set up incorrectly, MYOB will often not allow you to change that Item after it has been used in a Purchase or sale. You may need to create a new item, transfer the count and value to the new item and make the original item inactive.
Regards
Phil