Used Units are handled differently to New Units.
The Part number is automatically created when a Used Unit is purchased or Traded and then automatically deleted when the Used Unit is sold.
The part number is linked to the units in stock by having a Type of: Used Unit and the link set as the: Stock No. The part numbers for Used Units is created automatically and is in the format of .UCBR600-999999 where CBR600 is the model and 999999 is the stock number.
Please see Used Unit Schemes for the correct Tax rate