Take the price to buy it new today and subtract the cost of a tube, bearings, belts, rails, mirror, and lens. That should get you to a pretty fair market price. If your machine is really low hours/good condition, then you can be a higher. If it's been run hard, then I'd expect it on the lower side.
I know a few years ago VLS 6.40's with 50 W tubes were selling for about $20k. I also can tell you a tube recharge is about $1200 in the US and belts, bearings, and lens (what I'd expect to change) run about another $1000. So I'd start quite a bit higher than $16k. You can always go down in price if there is a lack of interest.