Just offer several ways to respond. I use PM, email, ph number. If I get no responses at all, not worth selling here (or anywhere else). Also, the one missed 'reason' from the above list IMO. There's been a developing trend in both online ads and at swap meets. Things are for sale because the seller is dead fu**in broke and truly needs the money so he's fair game for insulting lowball offers. It's been said to me in person, "Well, you need the money, right?" "Go **** yourself, NOT for sale now." is the answer.
All buyers? No, and we've actually done OK lately, but it's still out there. Sellers not responding? Not good business acumen. You're looking for that 1 person who needs what you have, but how do plan to reach them if you don't respond? When I don't get a response I move on, keep looking. Simple...