Go to ebay and find your 6600K in used condition.
Filter on completed auctions and you will see what the actual selling prices were.
Looks to me like one might fetch $125 if you were good at marketing.
If your chip has overclocked nicely, you might advertise that and get a better price.
As of 12/04/2016
What percent can get an overclock at a somewhat sane 1.40v Vcore.
If it has never been overclocked, that will appeal to some also.