It makes absolutely no difference if you are buying your Inventory for cash or credit, the Inventory Items tab works in exactly the same way. When you set up your Inventory you can also set your cost and sales values.
No, if you want to record your sales per item you can either list them in a book and then do a Cash on Hand - Receive Money for the daily total or list them individually in the Cash on Hand - Receive Money entry. If using the book you would record your cash and credit card sales in separate columns.
It is recommended that you have two Cash on Hand accounts to manage your cash and credit card transactions as this will provide you with better management controls.
This way your Sales - Cash balance should always equal the cash on hand or unbanked cash and the Sales - Credit card balance should equal the amount of card payments not yet received.