I'm another one of the folks who checks repeatedly through the day for messages and generally answers them promptly. And I absolutely agree that's good business practice for anyone whose intention is to be successful at what they're selling.
I also echo Claire that there are circumstances where that expectation must be relaxed and room made for events people can't always control. Many sellers on Etsy are SAHM's, and their primary responsibility is to their children, who could very well be passing a stomach bug around the family (as just happened with my son and DIL, so that everyone was down for days).
I think one of life's most difficult-to-learn lessons is that we are NOT in control of everything, and not everyone lives their lives the way we do. I just made a major sale to a woman who's a seller on Etsy and only checks her messages every few days. The way I would have liked it to transact? No. But I came to really like her and certainly appreciated her business.
Here's what I'd suggest you consider. You don't know whose taking care of an elderly family member with dementia, and can't get to the library. Nor do you know what other burdens another person carries. So treat them with compassion and respect, whether they respond in your timing or not, and I imagine they'll be grateful enough to do the same toward you.
If, for some reason, you absolutely have to have an answer in 24 hours, just say so, politely. For example, I'm buying a bunch of supplies right now to tide me over the summer, and they have to get here before I leave to drive north...or be sent north to my daughter's house. I just tell people the dilemma, and if they can send it in time for me to get it here, they let me know. And if they can't ship for 3 days, we work it out to send things north.
I'm old enough now to understand that life happens, and most people (not all) are doing their very best. If I feel a time crunch and they don't, it's not their responsibility to read my mind. It's mine to tell them.
If I tell them, and they still don't respond respectfully (which is true of very few sellers on Etsy, I've found), then there's the answer to your dilemma. They aren't someone you want to do business with.
A little flexibility in both directions could be a good idea.