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

I guess I’m not alone in thinking an offer is a starting point to a negotiation. I am happy to hear it.

I've sold a lot of things on eBay, very often with the 'Offer' option. I had one item listed at $995, it was probably worth $850-900. I never even saw one eager buyers first 6-7 offers, they were so low they were automatically ignored. He started at $225, then kept adding $10. Was I offended? Nope. I did contact him eventually, complete waste of time. My favorite ones are wannabe buyers contacting me to bitch about my 'way too high price', how it's not worth that, and will never sell. I make sure to let them know when it does, lol.

Some sellers are great to deal with, and some are plain rude.

In my opinion offers that are about 30% off the asking price are fair, and we can negotiate from there (both as a seller and a buyer.) One of the smoothest transactions I've ever had on audiogon was for an item priced at $3300. I offered $2700, seller countered with $3000 and he paid shipping and we had a deal within a few minutes.

Same as a seller. If you make a super low offer I politely decline. No need to get personal. Unless you're a scammer who "purchased" the item I sell and never pay for it, and ask me to send it to another country or your friend who's going to pay me later... 

So many sellers prices are inflated, I sell high but my equipment is always in perfect, as new condition. I personally pay more for that when I buy. I’ve also “lowballed” because the selling price was too high, not near market price, I’ve also purchased this offers as the seller realizes they are too high. Negotiations are a skill, so many just make rude comments and walk away if the price is too high. I encourage buyers to suss the seller out. You may find a great deal. People talk in absolutes in this hobby, there are no absolutes. Maybe people here should listen to music a little less and try to interact with humans a little more! It could be beneficial.

What you are running into may itself be a negotiating tactic. Acting as though you wouldn't consider and are insulted for having received such a "low ball offer" may just be another way of asking you or suggesting that you should increase your offer. Make whatever offer you feel comfortable with and it the seller doesn't like it you don't make the deal. The smart seller would try to negotiate but not all sellers feel the same.