Best way to sell old PC?

Solution
Rule of Thumb - Take your build / purchase cost and then subtract 10% .... then 5% for each month of age for 6 months (60% value at this point) ... then 10% for every 6 months thereafter.

0.5 year old - 60% of build cost
1.0 - 50%
1.5 - 40%
2.0 - 30%
2.5 - 20%
3.0 - 10%
Rule of Thumb - Take your build / purchase cost and then subtract 10% .... then 5% for each month of age for 6 months (60% value at this point) ... then 10% for every 6 months thereafter.

0.5 year old - 60% of build cost
1.0 - 50%
1.5 - 40%
2.0 - 30%
2.5 - 20%
3.0 - 10%
 
Solution
Best way is to find a friend who needs a PC, or post signs at local school. Agree with Jack above that you will not get much for it on ebay.

To find price on ebay, find a few comparable systems, note the price, see which sells. If they don't sell all you've learned is that the price they want is too high. If they sell then you know your price range.

It it boots, do the upgrade to win10.