To accurately calculate depreciation take the amount you paid for it, subtract the salvage value (if there is any) and divide that by the life of the computer. Say you originally paid $1K for the PC in September and you've owned it four years and there's a salvage value of $200.
($1000 - $200 / 4) * (4/12) = $266
Your system is worth $266. Now depending on the condition it's in add or subtract another $50 to get the selling price.