I have several comments.
1. You refer to your "young daughter" and baby-sitting money. You also state that it was your daughter who made the offer. Perhaps the seller inferred what I did -- that your daughter is under 18 and not even of age to be using ebay. (Ebay policy and the law don't allow minors to enter into contracts.) And if that's the case, the seller was within her rights in blocking.
2. If the offer wasn't accepted or declined within 24 hours, the offer is voided. It wouldn't have still be open after a day and a half.
3. Although it may be true that this is your daughter's first purchase for which she saved a long time, you can't imagine how many sob stories sellers hear from buyers with cancer, buyers who just lost a significant other, buyers who work so hard but they're home just burned down, etc. These are common stories from buyers looking for "charity donations." And most are lies.
4. Sellers can block any buyer, wannabe buyer or even a member who they're read about on a blog for whatever reason they want. If a buyer makes a seller uncomfortable for whatever reason, it's in that seller's best interest to avoid dealing with someone who makes them question the transaction.
As
@JadaStormy commented, it's possible that the b.o. was set by default and the seller didn't want her listing to have a b.o. option.
Rather than deal with offers at all, many sellers just set a fixed price and if it doesn't sell at that price, the seller will either keep the item or wait for the right buyer to come along. And if it's the case where the seller didn't even intend to have a b.o. listing, an offer of 25% off her price certainly might be considered excessive. (Remember, "reasonable" is subjective!)
Bottom line: There are lots of sellers with Hermes scarves. Find another seller.