Okay, I see now. Yes, $300 is a little cheap for the US, but not outrageously so. I paid $500 for a fully-complete with all original parts 128K a couple years ago, from its original owner.
Actually, the listing is ambiguous to whether the original board is present or not, as elsewhere in the listing it mentions "both" and "digital board" which I've got to presume the seller is calling the motherboard the "digital" board, which I see could be a logical deduction if the other board is called the "analog" board.
..and dealing with someone who would close the listing with "if my daddy says you live close enough".
OTOH, $300 for all that otherwise original, period, stuff isn't too horrible. sure, it could be less. Are original motherboards that hard to find?
I kept several over the years just because they were gonna be tossed otherwise.