not EVERYTHING listed in this forum is in "Auction" format.
There is NOTHING in the first post that says it is an auction. It would have HELPED had the Op selected a "Make offer", "Buy Now" or "auction" tag so was more clear what they were looking for.... but he/she did not "start" an auction (there are zero terms in the post saying when ends, min increments or starting bid etc) so is by default just listing "for sale" and taking offers..... and he/she is NOT obligated to accept your $100 offer.
clear up what the sale format is and good luck with your sales!