Who knows, the cat may be out of the bag. It wouldn't take a rocket scientist to Google for "rca trk-12" and learn in 60 seconds that it's a historic and valuable item. Among other things, that search will reveal this very discussion, where TV collectors are wetting their pantaloons over the thought of buying it.
They may be wising up as we speak, in short.
Under the circs, I see two options:
1. Make a low-ish, but reasonable and fair, offer that would let you turn a modest profit by immediately flipping it. Be prepared for rejection. (Under similar circumstances, I have made offers that I considered quite generous; the sellers instantly got dollar signs in their eyes and decided to look for bigger fish elsewhere.)
2. Step out of the deal and tell them it's historic and valuable. Advise that they list it on eBay and direct them to this discussion if they haven't already discovered it.
It's hard to keep secrets in a forum that's visible worldwide a few milliseconds after you click "Submit Reply."
Phil Nelson
http://antiqueradio.org/index.html