If you have the used computer as inventory with cost, and you add the components with cost then yes you  are  doubling cost.

Create the items, zero cost, zero qty

use inventory adjust, set the adjusting account to the cash bank account, and lower the computer item

Check the cash account regster to […]

Resolution for 'account regster issue: 'Product Transformation on Inventory' available: Yes.

The account regster issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the account regster issue yourself by looking for a resolution described in the article. If it is a complex account regster issue or you are unable to solve the issue, you may contact us by clicking here or by using other account regster support options.

For more information about resolving issues related to account regster in Quickbooks/QBO or to get support for the issue 'Product Transformation on Inventory', click the link below: