If the seller is processing the postage on line then it is automatically marked as posted.   Until the package is actually posted and scanned it will not show as being on the way.   It is just the system, and not the seller's fault.

I have had sellers "post" my item at midnight, just minutes after I have paid.

I really do not understand this preoccupation with tracking a parcel from the second it is paid for.   I just wait for it to arrive.