Best bet is to part it out. Go to ebay and log in and search by the exact item. Then filter the results to items sold and that will give you an idea. Don't forget ebay gets 10%.
I'd think around $300 is about right. GTX 770s seems to be going for around $75.
Hello ezeron, I think you must be visit a used hardware selling websites, there you can get a better price and quick response too. And i believe that your stuff will get a decent price.