According to long tradition, apart from the books of Tobit and Jonah, which, I believe, are the only books held to be apocryphal, the Old Testament is historical from the time of Abraham onwards. Consequently, not to believe in ghosts is not an option for Christians, since the words of scripture are binding; and there is no reason to believe that the story of Saul and the Witch of Endor is apocryphal and not historical.
Once you start denying the accounts of God's ordinances concerning mediums, ghosts or anything else, as expressly recounted in such historical chronicles, you may as well indulge any "pick-'n'-mix preferences you care to - as though the Bible were an ordinary book, written by authors of disputable authority; rather than as books inspired by the Holy Spirit. You would at least share this attitude to Judaeo-Christian scripture with some liberal theologians.
However, common-sense counts for a great deal, since though perhaps not as common as it mind sound, it is one of the most fundamental gifts of the Holy Spirit; the ability to prioritise. ("What does it profit a man..."). And common-sense should tell us that the conviction that ghosts exist and have interacted with human beings in some measure is probably as old and as widespread throughout all cultures, as religion, itself. It has been said that modern Western man is the only sorry remnant of mankind throughout history, not to have believed in God as the only plausible explanation for the origin of the physical universe.
I believe some ghosts present a very real appearance, and it is only afterwards that the person who saw him/her realise that the person had died before the apparition.
In my experience, it is mostly people who didn't believe in ghosts who have seen them and told me about them. Although I haven't personally seen a ghost, I heard one swearing volubly into a priest's microphone in a foreign accent, during Sunday mass at the local Cathedral.
Also, some months ago, a bizarre noise used to be heard coming from our TV, as if of a solid object clattering about, while the picture of live broadcasts would be held, shuddering slightly as if on a film that was trying to move on. Since modern TVs are solid state, with printed circuits, I had to take it to be a poltergeist or ghost. One day, it got really loud and insistent, in a kind of crescendo, so I took the occasion to leave the room, to go to the bathroom. On my way back, it suddenly occurred to me that I should be praying for the person's soul, so immediately began. Just as immediately it stopped. Although with a few brief threats/rumbles of starting up again a little later. I still try to remember to pray for him/her/them.
But when we didn't used to need 2 handsets to turn on the box, it would switch on quite loud in the middle of the night. Turning the front of the handset away from the set didn't help, unless it was held down by something, so presumably the ghost's strength to perform such things was limited. Another strange thing was that we had to do something else to get a channel - which I can't remember now - but this evidently bypassed that requirement.
I've had unmistakeable, tactile experiences too, but don't want to repeat them. I mentioned them on an earlier thread.
Incidentally, demons, devils are pure spirits and not the same as ghosts.