Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Regettably, the safe is not worth anything. Because there are several alternatives, and none of them are good enough for the safe to be worth anything:
1. without being the original owner, in possession of the original bill of sale, you cannot go to Sentry Safes and ask for a replacement key, or a re-issue of the factory set combination. They will not honor such a request from a second owner.
2. you could go to a locksmith, and perhaps the locksmith would know how to produce a key that would fit the safe. (I doubt it... but even if the locksmith did...) The locksmith's service charge would undoubtedly be greater than the value of the safe, or the price of buying a new one.
So I'm sorry to give you bad news, but the safe is not worth anything according to all my knowledge.
Incidentally, and in case it might give some hope of some other opinion: What is the model number of the safe? the serial number I cannot use, but the model number might suggest something.