its there mistake they overcharge for every thing ...
i know im a bad person but hey so are they
Two wrongs don't make a right, and how much a company charges does just justify stealing from them. If you were caught shoplifting and went before a judge and tried to tell him "it's the store's fault for charging so much" he'd lock you up and throw away the key.
Back to topic, as far as this situation is concerned... legality doesn't matter. If you keep the light, even if they have a legal right to expect it back, you'll probably never get caught. Legal or not legal, you need to decide what kind of person you want to be.
Imagine this scenario: you lost your wallet with $20 and an ID (and thus a way to contact you) in it. You later find that someone found it, and because they had no legal obligation to notify you and return it, they didn't. Would you think to yourself "Well, they had no legal obligation, so I guess good for them" or would you be even a little bit angry that they didn't make even the slightest effort to contact you?
I guess what I'm trying to say is that legal or not legal aside, and if you are a religious person, God aside, no amount of money can buy your conscience. You are the one who has to deal with your choice, and if you make what you perceive to be the wrong choice, the money you make selling that light will not help you. So whatever your choice, these are the moments that define character, so think long and hard about whatever you do