QB Issue Resolution:

you stock the inventory item, See this for a How To Receive Inventory Items. (desktop)
http://onsale-apparel.com/Rustler/tag/how2-rcv-inven

serial numbers are independent of the stock qty, the list can have more or less serial numbers than the qty on hand – just the way they did it.

so you sell the item and select or enter a serial number. When you purchase the item, QB creates an average cost and qty on hand and holds it in inventory asset, when you sell it, the qty goes down and the average cost is removed from inventory asset and posts to COGS

Inventory and assembly items have three accounts
expense = COGS
income = your sales income account
asset = inventory asset

Resolution for Issue 'The cost of an item does not follow the serial number of the item. Where do the Cost of Sales figures come from? It appears the only cost is from the purchase order and when you receive the item with a serial number there is no cost attached to the serial number.' available: Yes (Solved).
Source: Intuit Community forum.

The cost of an item does not follow the serial number of the item. Where do the Cost of Sales figures come from? It appears the only cost is from the purchase order and when you receive the item with a serial number there is no cost attached to the serial number.: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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