What I do like is when there is something popular and the people that send the generic message lose out over people who bothered to type something to move the sale along. So the answer is often "yes it is available" when they send that first generic message, but has often become "no, it's sold" by the time they get to the second. Making "is this available" a completely pointless question.
It does happen. And it does make sense to ask if it is an old listing, but when it has only just been listed it is a pointless question. It also doesn't take much to put something more in. How hard is something like "if this is still available I will offer you $xx and could pick up on the weekend"? If it is not available, they will tell you (or ghost you, as per their preference!) and if it is you have moved things along towards a sale.
You will also find that some sellers just won't reply to an "is this available" message because most of the time it is people just wasting your time. It's not uncommon to have maybe 1 in 10 or less that make that query actually follow up if you answer - even when you answer straight away. When you add in no-shows and other dodgy characters there are a lot of Marketplace sellers that are one "is this available" post away from just throwing their stuff in the bin instead.