I agree,12K is to much,I bought mine(a 97 crew 350) with 110k for 15 k,Ive done the ball joints and a tranny(it puked at 237k) and the vacuum pump and alternator bracket and water pump,it's got 250k on it now.The service manager at the dealership told me I was nuts for looking at a newer truck as the 7.3 is the best they've made.
Without even seeing the one your looking at and going off your description,it's at best a 8000 dollar truck,You know for a fact they gave the guy a 4000 dollar trade in.