You do not mention if the ad had a "buy it now" option or not. It it did, and the other person chose that option, you are out of luck. I agree with others here though. Too many times, I have had an eager potential buyer back out on a promise to buy. The first one to make a firm offer through the system, and have that offer accepted, gets the product.
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.