I actually like Gumtree. Sold a few things from there and one of them I thought would never sell due to oddity (Early Samsung TV 3D Glasses). Bought a few things from there too no problems.

However I have encountered the absolute opposite to low balling which is actually fairly common in the Sydney second hand market - Some dude trying to sell a 2nd hand item (regular, not collector worthy and common) at either at a retail cost or just a couple of bucks off. When you point it out, they refuse to shave off anything off and if they do, is only a couple of bucks.