That's likely why you found it on ebay.
The original 5D does not have live view, so its difficult to check the AF system, but it is the likely culprit. Either send it to Canon for a fix, or better yet, return it, since the cost of fixing it might exceed the worth of the camera, if their is major trouble. Even a minimal AF adjustment might run $200 or more.
The 5D can produce very sharp images.