Have a gunsmith look it over. If you want to inspect it yourself, take it down, including the trigger housing. Inspect the housing and receiver for signs of wear. Check the O rings and the stainless tube. Powder tends to cake in on it if the shotgun is not regularly cleaned. Check the wood and blueing for scratches and dings. As for price, do an Internet search for the shotgun in similar conditions. Be strict but just with your appreciation of the gun condition. A ding is a ding and a scratch is a scratch and rust is rust and like new is like new. Much simpler to take the shotgun to a gunsmith or your nearest gun store and have someone with the appropriate knowledge take a look at it. Otherwise you risk not knowing what your buying. Good luck.
BTW, I've had mine for over 15 years as my main hunting shotgun and I have no need or excuse to replace it.