You could make a valid argument that a GTX 670 has better raw performance than a $110 GTX 1050, but neither are considered great for anything but super casual gaming these days. If I were putting a HTPC together, I'd go for a 1050, hands down.
So unless someone is looking for another 670 to SLI, I doubt you're going to find too many eager buyers out there. I'd say sell a GTX 670 anywhere between $50 to $75. The faster you sell it, the better. Kepler is 3 generations old, mind you. I have a GTX 260 lying around that I should have sold years ago - now it isn't worth my time trying to sell.