I am considering selling my gaming pc right now. I currently have: MSI GTX 1070 8gb vram, intel i7-8700k, ASRock z370 Killer, Corsair Vengeance 8gb ddr4 ram, Samsung 850 evo 500gb SSD, 1TB Hard Drive, Corsair H100i v2 Dual fan liquid cooler.
Off the top of my head, I'd imagine you could get $1,100-$1,300 out of that.
For used? No. You will definitely be getting way less than that if you sell the system as a whole. More like $900 - $1000 at the minimum. You lose a lot of value on new hardware once its' taken out of the box.