Brand new, untouched, in the box, PowerTap C1 Chainring Power Meter, 52/36.
$500 SHIPPED. (Paypal "send to friends" option, otherwise add $20 to cover fees)
You need a compatible 5-bolt, 110BCD (compact) crankset for this to fit on.
I have a used, compatible Shimano 11-speed crank (RS500) I would be willing to sell separately for $60, if you need one.
Check Powertap's site for technical details, or to see if this will fit your crank:
These photos are stock photos, and the last 2 photos demonstrate what it looks like installed on a crank. You will need to furnish your own crank, as this listing is for the rings with powermeter only (unless you buy my crank with it.)