Could be a non-price issue generating the non-response. Just because something is for sale doesn’t mean you’d be willing to sell it to just any person. If someone is super new to the hobby or has bad feedback or seems unreliable (just some examples) then just ignoring their pm is fine in my book. If someone offers 1% of asking price i would assume they were just trolling.

If a buyer seems like just dealing with them will be a hassle then not responding to them is nicer than telling them they aren’t worthy of doing business with you for whatever reason.