If the cell phone or laptop are still repackaged without a sim card, etc. Is there a way to locate something like that?
No, because the battery should be inside and charged. And the Immei number should be known.
No
Does the seller / online shop know the Immei number when the goods are already with the customer? Or you can only see it on the laptop / packaging itself
No, for a location, a locatable radio signal must be activated on the device to be located, mobile radio, WLAN, Bluetooth or a combination thereof, and the device must be clearly identifiable.
If he is online then that would not work.
Some stores know the IMEI numbers of the devices they sell, others don't.