When individuals barter, they generally have a firm sense of underlying value, i.e. "What's this thing really worth to me?" A 10-year-old car might be worth $1,000, to judge from the Recycler or Craig's List. At $20,000, a used car is a bargain only if it is a 1949 Ferrari Spider with the original piping on the seats.
Cities, on the other hand, often appear not to have a sense of "beyond this price we will not go." True, they bargain for big things on which it is hard to pin values, such as stadiums for NFL football and professional soccer. Still, the fact that cities are willing to entertain highly aggressive offers suggests to me that some city officials have a hard time drawing a line between a good deal and a bad one.