The reseller is not providing you with correct information if you are asking about connecting the printer to the USB port on the AirPort Express.
It does not matter whether the AirPort is the "main" router or not on the network.
The USB port on the Express will only support printing.....if your printer is compatible with the Express.....which you will not know until you try it out on your HP device.
If you want to use copy, scan, maintnenance and other advanced features on the HP device, you cannot connect it to the AirPort Express.
If the reseller told you that copy, scan, maintenance, etc and other advanced features will work if you connect your HP device to the USB port on the AirPort Express......the advice that they offered is not correct.
You can easily prove this to yourself if you connect the HP device to the USB port on the AirPort Express and then try to scan. You will not be able to do so.