I'm wondering how will the government know if I am getting money for bitcoins when it is just cash deposits?
They can't, but if they are going to ask you about the nature of these deposits, you'll need to find some excuse, like selling used computer hardware..
They can still ask you to pay tax on any profits that you are making if you are selling used computer hardware.
I believe capital gains tax is low in America, maybe 28%. You only pay that on your profit, so if you bought at $200 and sold at $250, you would pay $14 per bitcoin tax, assuming that 28% number is right.
They are unlikely to know about it though, unless you buy and sell through coinbase, in which case they could find out about it as it is a regulated exchange now.