I local bike flipper I've bought a couple of bikes from has a firm price on everything. He gradually lowers the price every couple of weeks. He tells lowballers to wait awhile and if the bike is still available they can have it for that price.
That seems like the most sensible position for online sales.
I rarely haggle over online sales. Either the price is reasonable or it isn't. Either I want that doodad or I'm willing to wait another year or two for the same thing to show up at a price I'd prefer to pay. Besides, the seller has the hassle of packing and shipping. I don't want to be that nagging annoyance as the seller is deciding whether to bother with cutting and taping together cardboard corner protectors, or using an entire roll of cellophane wrap to be sure those odds and ends are secured against bouncing around inside the box.
But in person, sure, I'll haggle after seeing the item. Just showing up demonstrates sufficient interest. In meatspace experienced sellers and buyers can work things out.