Sales Person is not the same thing as logged in employee. To enable this feature search for "Select Sales Person During Sale" in Store Config and Enable it. See Store Configuration Options
Now when an Employee is logged in they will be allowed to switch sales person by clicking on Sales person and selecting a Name from the dropdown list. They will still be logged in as the same user but the sale will be recorded as if the sale were completed by a different user.
For security reasons the system always logs who is logged in at the time of sale. reports will display both who is logged in and the sales person that was used for the sale.