Where can I sell or trade a used Nikon d3000 camera?

The lens work fine and so does the camera.

The power Button's damaged, meaning it'll reset itself if I turn it off, and it'll stay on.

Craigslist: Make sure you meet up in a public area where everyone can see you.

Craigslist

EBay (to sell), or Craigslist (to sell/trade)
You could also try selling it to a camera store with a used department, like Adorama, B&H, or KEH
Make sure that you clearly state what is wrong with it, and that the camera is still usable.