Well two things are wrong with that deal, they didn't put 8" rears behind K codes ever, and the ratios didn't go below 3.50, others were 3.89 or 4.11. K codes got 9"( well truth be known 8 3/4" ring gears in 9" cases). If it is an 8" 3.00 posi it most likely came behind either a vintage GT Mustang-maybe that was what he was thinking, or out of a newer car-perhaps a Mustang 2, make sure you know what a posi looks like is my best advice, the 8" units are harder to find than the 9", and a $150 is a good price for a "posi" 8". Watch out for a "homemade" posi rear too.
1967 S code GT Deluxe Coupe 65B
1966 Convertible 76B 289 4 speed
1964 Fairlane 500 2DR HT-the new project
1986 Jeep Cherokee 4X4 the "beater"
http://albums.photopoint.com/j/View?u=1573428&a=11959254&p=43023698.jpg