If the case is not yellowed, the Mac has never been exposed to sunlight. Selective yellowing is unusual; tanned back and grey/beige fronts are common. Venetian blinds may create strange patterns. Is it yellowing or a stain?
Ask the buyer to look for the serial number. Is it on the bottom or next to the brightness switch?
The serial[de-9] to ADB adapter sounds like a cable adapter for an ImageWriter.
The carrying bag might be interesting. If the Mac Plus is a 128K or 512K which has been upgraded, it is definitely interesting. The 800KB drive is very sellable. Which keyboard is offered? Have a look at the power cords.