If you're getting a new X, your best value for your 3 may come from Tesla itself, on 12/31 around 7pm when they're desperate to sell any remaining vehicles inside 2018. Of course Tesla may look at you as being desperate to obtain the full $7500 at that hour too, so maybe a push. But anyway, perhaps worth considering (vs private sale) since Tesla may be willing to play a game or two with you when 2018 only has hours remaining in it.