Why do so many sellers take offense to offers on their listings?

I treat reasonable offers from potential end users reasonably. Those can maintain an intelligent conversation about a product, and tend to ask relevant questions. Sizable discounts may go to those who deeply understand the product.

I dislike lowballing from those who just want to make a quick buck on resell. Those don't care about the product at all, but just mechanically offer a low enough price virtually guaranteeing a profit on resell.

People get too emotionally invested in the gear.

People frequently overvalue what they have.

No one is forcing you to sell at the lower price. Be a grown up and ignore it if you don’t like it.

It is business after all not personal.

Unfortunately there are resellers who will offer you an amount that guarantees that they will make a massive profit when they sell at your current asking price.

I think it's fair to call these lowball offers and not develop kind feelings for them. 

"If you haven't noticed the audio market seems to have come to a standstill" 

No, I have not.

Whatever I am buying. If the initial asking price is not realistic I move on.

If the asking price is over what I think is a fair price, based on model, age and condition, then I assume the seller has built in a buffer so I offer a little under his price and see where it goes.

If the price is fair, I don't quibble and pay the asking price so I don't screw myself over a few dollars.

On average, I believe more problems start with the seller, than the buyer.

@tony1954

nailed it ! Point, set, and match …in tennis jargon.

But with a qualifier IMO that problems created in ad communication is not necessarily tilted more towards some Sellers,, but rather equal in number and in poor form with unrealistic 1st steps of some Buyers .