supply and demand doesnt dictate 28K for an xB. in order for that to happen, they would have to be way more popular than they are, and way more rare than they are. xB's are everywhere. its not like they are on order only. and, for that matter, neither is the tC. 18K for a used tC private party is a bit high, but not unrealistic. with some options on it, none the less. retailed for about 18,800 new.
thing is, you cant account for tax and license in your resale price. the next guy needs to pay it, as well. priced down at 16,900 it would be perfect. i assure you, its a little better than what i would give for it on a trade-in at my shop. then we would price it at 19, and negotiate it down a bit to sell it.