Do I need to stamp e-ticket before on board train?
No, Swiss e-tickets do not have to be stamped.
If you bought by any chance a ticket from SNCF, that requires pick-up from the SNCF ticket desk before travel, you are required to do so.
RolandP Thanks. Then if I buy a standard e-ticket which is valid normally for one day, without stamping, how can inspector knows it is the first time to use it in that day?