Lowballers
If I post something for sale, I tell 'em up front: It's negotiable, or it isn't.
If it isn't, and you try, the price goes up. I price my stuff fair and once the price starts going up...it will NOT go back down.
If it is negotiable, you have to come look at it before you're allowed to make an offer. (pics suffice for 'net deals). Give me a good reason for dropping the price (My wife died, my dog left, it's dirty, it's not dirty enough, it's ugly, my mom drove a chevy, my mom is ugly, whatever), and I'll haggle accordingly.
And if I go to buy something (I don't make offers without looking at it) and I feel like lowballing, I pull out a pre-counted wad of cash of what I'm offering. No haggling first. I mean business, dammit.