Shift is a new internet-based way to sell your car (likely reaping you much more money than you'd get in a trade in) or buy a used car (likely with a better buying experience than you get from a dealer). Unfortunately, it is not available in Phoenix. It is available in DC, LA, SF and elsewhere. If it ever gets to Phoenix, we will have our famed Car and Driver (CAD) Department evaluate Shift. In the meantime, the link below can it explain it all to you.
Link to Shift Website