Whelp, I got my brand-spanking new Spark EV with 133 miles on it appraised at Carmax a couple of days ago out of curiosity. A $27,135 MSRP 2LT Spark EV is worth.....$12.5k as a trade-in (another $750 if you trade in for a new vehicle at the same dealership due to tax savings).
I emailed Beepi about the Carmax offer, as they have a "we'll beat any brick and mortar dealership's quote!" guarantee on their website. After initially saying they would not be able to match Carmax's offer, due to the Spark EV being "primarily a West coast market car", I mentioned their "beat any dealer offer" guarantee. They came back a little later offering $12.6k.
So as a trade-in, it seems $12.6k might be the ceiling for a '16. If you consider a brand new '16 Spark EV 2LT can be had for at least $4k off MSRP, 27.1k - $4k dealer discount - $7.5k fed credit - $2.3k MD credit = $13.3k, the $12.6k trade in offer makes sense.