Ok, first of all, try to sell the camera to her PARENTS. The sale of an item is a contract. Minors cannot enter into contracts. If you enter into a contract with a minor it is entirely voidable by that minor. This means if you sell your camera to a minor (like her) and she destroys it, she can LEGALLY give the camera back to you and demand her money back, and you would have to give it back to her because contracts entered into with a minor are voidable by the minor. If you sell it to her, sell it to her parents instead to avoid this. These contract laws are in place to protect adults from taking advantage of minors in contracts. If the minor can make the contract void on a whim, an adult can't possibly take advantage of that minor through the contract. This also means that any person (including you) unlucky enough to enter into a contract with a minor can get majorly screwed by the minor.
Hopefully when you deal with her parents you can get a better method of payment like personal check or something, not to mention protection under the law in case the payment bounces, the cash is counterfeit, etc.