You enter Items (DESKTOP) or Products/Services (QBO) as charges to your customers on invoices and sales receipts. Set up Items you sell (or services you render) and select an income account that they will post to. Then what your customers are charged will be income to you when paid (cash basis) or when billed (accrual basis). It is that simple.

If you sell products you buy and hold in inventory these will be two sided as you will record same item being purchased will go into current inventory asset but against cost if goods sold when you sell