I have recently seen new-in-the-box ones ranging from $175 to $200.
Even though yours has never been on a weapon, you will be hard to find someone paying a "new" price for it when they can buy one they know is new.
I would price it somewhere between $150 and $170 if you want to sell it.
Hope this helps. If you have it priced for awhile and no one bites, drop the price incrementally until it sells or until you decide to keep it.
Or you could spend another $50 and get a mil-spec buffer tube and keep it yourself.