Agree w/ PB, when you buy used, you're always taking a chance that everything will turnout alright. I doubt the dealership noticed there was a problem with the sensors when it was sold. Most of the time they do little else on the car except detail it. Unless its a certified used car, then it goes through an in depth inspection, and comes with a warranty.
Since it wasn't certified, that means this issue is on you. It was your responsibility to check the car out properly before signing on the dotted line. That includes testing out all the accessories, including back up sensors. Don't feel bad, many people don't do a good job of that when buying used.
Unfortunately buying an extended warranty wasn't a good idea either. Obviously extended warranties are big business for dealerships because most cars never experience a major problem until after warranty has expired.
I have a 2011 without the sensors, and would have foregone them in this case, as opposed to paying $900.
I'm sounding like a *****, but I'm really not trying to, that's just how I see it. Good luck the rest of the way.