I tend to do a complete build every two years but upgrade in between, awaiting a new case and fan controller and a couple of additions to my Prodigy build (Side window and mesh front panel) as I type. I have no problems selling on my components after an upgrade etc on another forum I am a member of.
But I cannot remember when I have sold a complete PC as I will make more in breaking them down and selling the components individually. I will add that it is still a loss making venture.
Us gamers will still want our desktop PC's and I cannot see that changing anytime soon.