Having had similar experiences, I share your frustration. You sound like a serious buyer and the seller has not treated you well. However, "a good convo with the seller" doesn't replace a firm offer. Your story didn't say that you had submitted an offer, but that you believed the seller had afforded you the opportunity to commit by the end of the day. In the absence of an offer, a seller can get frustrated and sell to a firm buyer, even below the asking price, if he thinks that a potential buyer is just tire kicking. It's best to just move on and try to avoid that "highly graded seller" in the future.
Losing out
I'm a little ticked. Was having a good convo with a seller and was afforded the opportunity to respond/commit to buy by the end of the day. I did. Lo and behold. Sold to another. Wasn't given a courtesy heads up that an offer had come in even though I was within the time frame noted. Am I wrong to be irritated? I don't operate that way. Even with people I don't know I do the right thing. Don't believe the right thing was done today by a highly graded seller.