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

When I sell a used item, I mark the price as FIRM. My prices are almost always lower than others being currently offered. When somebody lowballs after that, I find it very annoying. It’s hard not to be rude to the low-ballers given those facts.

@secretguy 

I would insist on freight shipping if you are dealing with floor standing speakers with cardboard packaging. Unless they are in wooden crates, they are unlikely to survive FedEx / UPS delivery. 

I've been shocked a bunch of times, at the low price an item I was watching, sold for (relative to asking price). Would have bought it if I knew how low price would go. I'm never offended by low offers, except the few that then argue why I should accept their low offer. Most of us here are not professional sellers.

Value is in the Eye of the owner. And sometimes we get emotionally attached as human beings. 

I’ve seen on USAM that there are people who seem to just low-ball everything. Occasionally they get a taker, and then the re-sell it here. Supplemental income, I suppose. Depressing. I guess you could call it arbitrage or market clearing.