Howdy Pard,
I would pick it up and tell them you are having second thoughts. If you sell, the best place to sell a Spencer is on Gunbroker.com Any Spencer gets $1400-1500 easy. That way you control the sale. You do not need an FFL, nor does the buyer, as i9t is an antique, pre-1898.
If you can do things like tell the markings on top of the receiver, the barrel length (20" or 22"), etc. can tell the caliber. If it is 22" from muzzel to breech, and is NOT marked "Burnside" on the receiver, it is an early 56-56 Civil War vintage. If it has a 20" barrel it is probably a 56-50 Indian Wars version.
If you email me JPGs of both sides of the receiver, the top of the receiver, and overall, I can give you an idea of what you could get for it. I am not really an expert on Spencers, but I have owned and shot two originals and two replicas. I have also written two articles about the Spencer. Orignals make good shooter, and are not hard to convert to centerfire.
Email me direct <imarangemaster@yahoo.com>
Tulumne Lawman