Three times yesterday I sent replies to various ads to purchase items at the sellers asking price. NOT ONE BOTHERED TO REPLY.
Now the occasional non-response is typical of ham selling, but three in one day seems to be the top of the heap. Perhaps it's the absence of face to face dealing which makes it OK to throw manners out the window. But we all know that politeness quickly disappears in the anonymous internet environment.
If you post an item for sale and receive replies, then IMHO you are obligated to respond in some way. Yes it's a pain in the butt to have to reply to a large number of offers or questions. That is part of selling. Even at the very least amend your post to say "Sorry Sold Thanks to all who responded" which will cut off any new replies.
But if you have a bunch in your Inbox , you owe each and every one a reply of some sort.