That doesn't explain why people have done it on Craigslist since before Facebook existed, where they have to open an email client, type it out, and send it.
Sure, sometimes people leave ads open after the item is no longer available. But only asking if it's available is still an obnoxious waste of time. The first message from a potential buyer should have something useful in it. Further contact info, meetup availability, clarifying questions, an offer if the price isn't firm, etc.
Maybe lead with, "If this is still available, blah blah blah" if it makes the buyer feel better. The buyer probably has all that in mind when they decide to contact the seller anyway, so they can take 30 seconds to include it in the first message and actually get the process moving instead of holding it up for a one-word reply from the seller.
If you buy enough items that stale ads are actually taking up a meaningful amount of your time, then copy-paste as needed.
"If the post is still up, the item is still available. I will remove the post as soon as it is no longer available. If you message me and ask me if it's available, I will block you."
It nice to hear from you.well am pretty okay with your asking price but i just need some re assurance on the condition,am buying it as a gift for my son in United Kingdom so can you accept paypal as a mode of payment and i will add $90 for shipping,i will have do it local transaction but i am out of town presently..so you can get me back with your paypal email account so i can pay now.hope to hear from you soon.kindly text back ASAP