The market "over the counter" is composed of an electronic network, where the parties negotiated financial contracts. These operations are conducted between banks, brokers, financial institutions and corporate clients.
It is important to be clear that in penny stock investments are high risk, since companies where it gets the money can be a long time with the same quote, either break.
But it is also true that you can buy a stock at $0.001, and then the same raise to $5 or more. This event we could get thousands and thousands of dollars with a very low investment.