The private key is like your password so you should not give this key to anyone because someone who holds the key is the one who holds the coin number in your wallet. 

I can understand your problem this way when you buy the bitcoin price at that time is $ 500 after you buy the price it will be $ 300 because bitcoin is a coin unlike gold or any money Anyway it's thirsty it can fluctuate quite large price in short time. This should draw experience when buying bitcoin should see where it is if it is too high, do not buy and you will buy it at a lower price. Good luck