If I am not wrong, most people were able to mine Bitcoin with their CPU back then. I don't know exactly when the mining moved to GPU hardware and then ASICS, but I heard that it was possible to mine Bitcoin through CPU power. How else do you think people mined the blocks? I don't think Satoshi sold any of his mined Bitcoin back then. Some early miners sold their Bitcoin for a very low price.

As another member already said, there were already some exchanges. So, possibly some people also bought Bitcoin at that time. But mostly people acquired their Bitcoin for free. I wish I knew about Bitcoin back then.