Hard to say. Generally, sellers of Zeros I've noticed want more than buyers are willing to pay. If yours is still under warranty, you can demand higher price. But be prepared for low-ballers. It also depends on whether there are incentives on new bikes in your state. When I was looking to buy a '15 S or SR, there was $1k incentive from Zero (a cash card), Marylabd had $125 per kilowatt battery capacity, and there was a Federal tax credit. With all that, I could buy a brand new S for less than what many folks were trying to sell used. So I used that to negotiate on price and bought an almost new used '15 SR with a 12.5kw battery for the cost of a new S with a smaller 9.5kw battery. Folks in some other states did not have state incentives, so were willing to spend more on a used bike than I was.
Sorry, can't give you a high-confidence number, but I think you will be lucky to get $8k for yours. Check cycletrader asking prices as a guide, thoigh I think they are generally too high for the used bikes. As with all electric vehicles, they tend to depreciate quickly, but Zero I think have been holding their value better than say a Nissan Leaf ...