Steve should've listed it here!
$500 isn't bad for the 10" Bintel (it's the same one I have), they're about $800 brand new, but it depends what quality it is in and how much postage is. Cause you might be better off buying a new one from Bintel or Andrews.
I'm not sure about the quality of the skywatcher, but Steve is a nice guy and so should do you right should you enquire abotu it.