As Hunter mentioned, YES you can input sales that are not from eBay manually, as long as all of the required fields are filled in.
The obvious ones of course are your cost of goods, shipping costs, how much you sold it for, etc...
The important one that is not obvious though is the Item/Transaction ID(unique) which is in Column B and hidden by default. It is populated by an always unique value from eBay and the sheet uses it internally in several processes.
To display the hidden column(s) turn ON the "click layout assist" button.
Click the double triangle above the "Listing" blue vertical bookmark column separator.
If you want to permanently display the Item/Transaction ID(unique) column, delete the "h" above that column.
Click the double triange again to rehide the unwanted columns.
Turn OFF the "click layout assist button" (should be kept off when not in use, as it can slightly affect the sheet's responsiveness/speed).
You can fill this column in with any value, just make sure it is always unique (not duplicated with another row). So you can make them 100000, 100001, 100002, (etc).