I don't always have a lot of stuff posted for sale, most people haggle fairly and I come back with a bottom dollar, they are fine. I have had an occasional dumb dumb ask if I would take next to nothing for an item, I politely respond no, but the best I can do is this, they messaged back and offered a small amount over what they first said and that is when I hit the block button and move on. I know it would be most satisfying to be able to smack them in the face and call them an idiot, but them not being able to contact and bug you again is the next best thing.
I'm having the same issue on Craigslist. I'm getting lowballed to death by someone who says "I can buy it brand new for the same or less elsewhere". That was in an email - never even came to LOOK at my item - so I wrote back that if you can get it someplace else for the same or less, why you shopping on Craigslist?. Of course, there was NO response to that..... What the hell is wrong with people these days? I mean... if I'm going to give my stuff away, I'll give it to charity.