On the OP's topic, your choices are to fulfill the order, or cancel and risk the account ding. You could message the buyer, thank them for the order, and let them know you want to make sure they are aware the product is past the best buy date. Let them know if they did not realize that and impacts their decision to buy, you would be happy to cancel for them and to let you know.
The other thing you could do is respond to the message you ignore. If it was just yesterday, you could say something like, "Hi, I apologize that I am only now getting back to you. I saw your message and set out to research how much of a deal I could give you, but some other things came up and I didn't get back to you. I am so sorry to tell you, but in the meantime someone came along and bought all 8 packages before I could respond to you. For the record, I would have been happy to do the deal you suggested. Please accept my apologies that I didn't get back in time."
By suggesting you would have done the deal, you may get this buyer to admit that they bought them under the other account. They probably won't say it was them, they will probably it was their wife who made an account and got them or something. At that point, they will probably want that deal and you may then get them to ask you to cancel so they can rebuy under the new deal. Of course, once you cancelled, you could just block both user names and for heaven's sake, leave the product down for a while so they can't make another account and rebuy.
By the way, the policy does not go into this much detail about this on eBay's website, but as someone who used to sell mostly health supplements and some food products, I can tell you that not only are you not supposed to list expired/best buy products, eBay considers the expiration date to be at the start of the month listed on the package, not the end. For instance, I have Keto supplements that will expire in August 2025. I will pull the remaining stock on July 31st.
There are exceptions to the expiration date policy. If you are listing a food product that is clearly a collectible item, rather than meant to be consumed, the expiration date rule does not apply as long as you list it in an appropriate category that does not imply it is meant to be consumed. You also should clearly state it is not meant to be consumed.
