Old school Phantoms are great fun, probably worth about £30/50 range for an old one, double to triple that for a modern version depending on what options it has.
Old school versions are fixed barrelled and came in 3 lengths, 8" Shadow, 11" Tourney, 14" Express.
The internals were nickel plated standard Nelson lay out, very easy to maintain.
CCi are still going and getting parts from the US is very easy.
I've an old school version but have replaced the internals with a modern valve, hammer and bolt.
Good fun but the Hammer is duel feed and will accept FS rounds.
If the one you were offered was the one on eBay it looked rough and the 12g quick change looks like someone has modified it to a rear lever change for some reason and it had lots of anno wear along the pump, shows that its had a hard life.