For common items there is an average price they sell for. That is "the going rate". To sell a common item, it has to stand out among all the others, or it will sit for months unsold because of the competition. So its price or your guarantee or your shipping have to be better than the others.
The problem is there are buyers like the OP encountered, that want it all. A "low ball" offer is any at half your asking price, if you are at the going rate. They think that "the buyer is always right" and that a seller should be either intimidated by their unreasonable requests, or just give in out of frustration. They have lots of time on their hands to surf and constantly try to find someone who will sell that $40 camera for $10. They may ask 50 people a month, but eventually get one.
It's different with uncommon or rare items. There are fewer comparable items so you will sell faster. You do not need to lower your price or offer free shipping. But some think they are in the same situation as the common item. I used to sell a lot, but have gotten out of it. I had a guy on a lens group post that he wanted a particular 1800s French wetplate lens. I connected, and he gave me an offer about half the going rate. When I told him they sell for double that, and that I wasn't planning to sell, but figured I'd help him out, he said "what's a few hundred among friends?" He tried cajoling me, bullying me, lying to me. Said "Oh...sorry, I just don't know anything about these old brass lenses, what are they worth?" I looked him up, and he's been buying and selling antique brass and wood cameras for many years, including that maker. Has shelves full of Alpas with Angenieux lenses too, dozens. Deal was off, and he's on the "I will NEVER sell to you" list.
Too many con men out there trying to rip people off on eBay.