14900 is a Dogleg Constellation, 14700 sounds about right though.
There's
a lot of gunk on it. I'd take a cotton swab, put it in alcohol and gently try to remove the worst of the gunk. Then, go to a watchmaker and just ask him to open it, so you can take pictures of the movement (showing the caliber- and the serial number) and the inside of the case back. Make sure those numbers are legible in the pictures. Coming from an unknown seller (you; no offense, really, but you're not a dealer/known collector, which does make a difference for pricing), I'd expect something like 600-700GBP in a no-reserve auction. Let the free market work its magic.
Slightly different configuration, but my own 14700 says hi:
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