All offers are valid. There's no such thing as a low-ball offer. Sellers use that term to describe a price they would not entertain. So what? Decline the offer and move on. No need to be offended. The flip side of this would be a seller's unreasonably high asking price. Potential buyers get all mad about it. So what? Make an offer or don't, then move on.
Yeah, I think people try to look at a single person selling a single item on a website and want to apply the same logic and transaction flow as if it were Guitar Center and they have a warehouse full of that particular bass or pedal. Make an offer and see what happens, get an offer, consider it and say "yes" or "no". It take less time to reply with "no thanks" than to stew over the low offer and post about it two weeks later. As a seller of one bass, you only need to find one buyer. You don't need to sell it 20 times to twenty different buyers. As a buyer, if the price isn't what you want to pay, don't. In both cases, move on. It will eventually sell or you will find another from another seller.
