I assume that most mom and pop investors will use the same exchange to buy their bitcoin and then later sell them. In that case the IRS can see both transactions in their account and know the capital profit/loss they have not declared.

If you buy at one exchange and then transfer the coins to another to sell it would be harder for the IRS to match up and discover your profit/loss. Especially if you move the coins via several intermediaries and sell in a different country that is not under the IRS jurisdiction.