seller etiquette

It's not 20 years ago. Today, everyone has a phone with them for every move they make. If you don't respond in a timely manner to a text or an email these days, it's because you are lazy or uninterested. Of course there are some exceptions, but those are outliers. I've had messages to sellers go unanswered for days. That's unacceptable if your goal is to actually move something.

Maybe the seller should have just told the second buyer to pound sand. But he didn't and did actually try to give the OP a chance at the item. 

If you can't get there the same day, offer to send a deposit via Paypal, Venmo, or whatever. 
End of issue. 

I've had so many people respond to ads saying 'I'll buy it', then you never hear from them again.

I had an experience with a bloke in Canada, from a well known Canadian retailer with a popular YOUTUBE channel. We went back and forth, agreed a price, he wouldn't use PayPal, so I sent him my bank details, then never heard from him again. Wanker.

Another issue is that email from USaudiomart is problematic and often ends up in a spam folder - not just me, there's various threads about the issue on their forum.

So if you're involved in a transaction as buyer or seller, be sure to check your spam folder.
Anyway, I don't think the seller guy did much wrong. He gave the buyer the opportunity to pay, he didn't respond, the seller sold it to someone who paid. 

Turn on your doom scroll radar to a level that detects items that has a high sale value. If the buzzer had gone off, you would’ve been much more active In acquiring the item. Pretty much know when you can sit on an item and watch the price go down and at other times the minute an item pops you’re making a phone call making your way down there as fast as you can. Still amazed that a used subwoofer got that much action in the classified but oh well. 

@gkelly it's been listed for 75 days, 5 views a day. So I did not think it was a hot item. I wasn't not active or very active, I was reasonably confident I was going to buy it. I had the money, and a place and time in 3 days. 

As to why I didn't read the email between 5PM and 6AM the next day: it was Saturday PM. I am fine with not reading email on a Saturday evening. I didn't think the seller would back out and sell to someone else. But it happened. 

The moral of the story: I should not assume anything. The end.

From what I’ve read, I agree with the OP. Particularly learning about the timing of all this, Saturday evening through 6 AM Sunday morning? All that said, it would have been Better for the buyer and seller to have exchanged phone numbers. Of course, that cannot happen on this website. Which is a whole other disappointment!