well usually i figure how much brand new then take a quarter to a half off if the item itself is in perfect condition or check ebay see what they go for on there then offer that much
another plan is ask the seller their min and ask myself am i happy to pay that
I wouldn't buy it, you'd be better off with a high-end air cooler than that. If your going to do water, do it right and get yourself a nice kit or build one yourself.