How much to pay for used Nikon D3100?

The seller bought it in January for $515, and his selling price is $400. Considering that it's used, shouldn't the price be less than $100 off?

Pricing is decided by either of two factors:
1. A linear depreciation on time owned
2. Depreciation on replacement value

Since the camera is still being actively sold in the market, apply method 1.
Current price on Amazon.com = $447
Depreciation per year = 30%
Since half the year is over, depreciation amount is $67
You can ask for deduction of a few more dollars towards cosmetic damages if any.

Fair price seems to be approx. $350 - $370/