Smee, I am somewhat philosophical about the communication process - to a point. As you rightly say, there are some things that aren't as efficiently maintained as they should - and communication does suffer.
I would give a 24 hour period to allow for a full day's cycle of the seller's routine, in case they check emails only once a day. This means they DO need to check at least once a day and would, thus, have some measure of organisation in their business.
HOWEVER, if a seller were to respond in the manner you do, Smee, they would certainly demonstrate a quality that would encourage my confidence in their ability to look after me.... and they would likely get my custom.
The Countessa's example is a case in point.
At the 12 hour mark, a purchasing decision had been made - and actioned - based on effective communication from one seller.
You might say 12 hours is still too long, but from a buyer's perspective, our expectations have been lowered sufficiently to endure it.
From the seller's point of view, though, every second they delay replying is one more second they give their competition to beat them to the sale.
You'd think there would be more sellers who are conscious of this - but I might suggest one reason why they don't is that they are looking at the internet as 'easy money'. Just set up your items and sit back (or play golf or go surfing), waiting for the money. It is, perhaps, one of the greatest strengths and yet one of the greatest failings of internet sales ... it's just too easy. The work ethic essential in the running of a B&M store should be applied to an internet business - but people get sales without it being particularly strong.
From my observations, you can tell the difference between someone 'selling some stuff on the net' and those running an internet based business.
Repeat business is often a good barometer. Consistently good feedback is another.
But the one that matters most at the time of imminent purchase - is answering the customer's question ... before anyone else does.