If it says 'price firm', I figure whether I want it. Usually, I don't, as often they're people who are clueless about the real value of the item.
When I sell things, I often meet them in the parking lot. Sometimes the item is too large....although, arcade cabinets and such are too big for most interested in theft, so I don't worry too much.
As for cheapsters: I was selling a decent desk. A college student came with her dad (TM), and he tried to lower the price I had already lowered before they responded to the ad. I told them, 'Either you want it, or you don't', and stared 'em down.
I don't think I'm ever wrong on the value of the item, but I ain't hurtin ta sell it, either.