Calculated Columns can only include other Columns with a Number or Currency format. If you are unable to select a Purchaser or Vendor column when creating a Calculated Column, double check the current format.
Navigate to the column you wish to include in your Calculated Column. Click on the edit icon.
The Edit Vendor Column window indicates the current format. In this example the column is set to a Text format and therefor cannot be included in a Calculated Column.
You will have to create a new column and set the format to either Number or Currency. Once you create your new column, delete the column that will no longer be used.
Now that you have created a Number or Currency formatted column, you will be able to select it when creating a formula for your Calculated Column.
For more info, please see our Calculated BidTable Columns page.