If you just used it for gaming/windows the card should have more life left in it. I have run cards longer and harder then you did with your GTX 1080. So I would say you could get $375-425 as the card should have another 2+ years of life assuming it is handled well and thermals are decent on it. Now had you mined with it or used some other operation that taxed the GPU 100% for more then 12 hours a day the story would be different.