I've heard that for decades the government has not tracked capital gains on real estate transactions. Instead they hassle the middle class wage earner who was W-2's filed but the millionaires who buy and trade real estate are left to an "honor" system.
The government does not audit particular kinds of claims, but returns in their entirety.
The selection criteria for an audit depends more on how the information provided compares to other information the IRS already has.
If there are inconsistencies, they initiate an audit.