by default, eCart uses the ID column of the orders table as the order number, the database controls this number, it uses an auto incrementing value so that each ID is unique.
you could edit the order table so that the ID starts at a different number than 1:
ALTER TABLE tblName AUTO_INCREMENT = 12000
this would start the ID numbers at 12000 and each order placed would increment from there.
using a bar code in the emails would take custom coding to accomplish and is not supported.