What would you pay/whats it worth

Haggerty's price valuation tool doesn't even list a Duster in 70 or a /6 until 74-5. The '74 /6 avg sale value 7,500 this would be actual up to date auction prices.
A 71 318 in cond 3 to 4 range from 6,100 to 8,200.

Good points are the car appears in overall good shape, is desirable body style, and the paint and color looks in good condition. As others have said you can adjust your price down through bargaining but it is harder to go up after the fact.