If you solicit offers, shouldn't you respond?

I recently made a very reasonable offer on an item that was being sold on a strictly "make an offer" basis (there was no option to buy the unit at the asking price, just "make an offer". So I offered the asking price less shipping).

The seller never responded, so I sent a PM. No response to that either. My offer has expired with no word from the seller.

Isn't there, simply as a matter of courtesy and responsible behavior, an obligation to respond to reasonable offers if you're soliciting them?