One year of sitting doesn't mean a thing to a Kbike, except for the battery and the fuel. I left a Kbike outside for over three years. New $50 battery, put some fresh fuel it it and it fired right up without a sputter. Spewed some black smoke for a minute, but that's about it.
If it looks like it's been garaged and well cared-for, if won't roll when in gear (indicates that none of the gear splines aren't totally not stripped out), if all the brakes hold pressure, and it looks like the battery is just dead when the key is turned, I agree with mlytle that it's worth more than a crappy-looking project bike, which is what you normally get for $600-$800.
Problem with lowballing off the bat is that you run the risk of pissing the seller off, after which point they'll refuse to talk to you at all. I've hung up on and refused to sell to rude low-ballers before. I'd rather just put it up on eBay and let people bid that have to deal one-on-one with people who want your stuff but don't want to pay for it.