If you solicit offers, shouldn't you respond?

Etnier, don't give up. A reasonable offer such as yours deserves a response; its that simple. At minimum, a simple "sold" response is warranted. If one posts a "for sale" ad in a forum such as this, there is a certain amount of responsibility to reasonable prospective buyers to not waste their time by forcing them to make repeat offers or attempts to communicate. My rule of thumb is to not buy from a seller who is clueless about simple communication etiquette; it's an indication of other potential problems.

On a related note, it's like the person who asks for advise about an audio-related issue, receives many well-intended advise/suggestions and is never heard from again. Nice.

Good manners is always a good idea. But it’s a bad idea to let people without them to ruin your day.

It is not a matter of being offended, or upset by rudeness. I agree it is simple good manners to reply and courtesy and manners matter. I would reply courteously to a ludicrously low offer even.