TheTaxBook has a good reference page.

If they're flipping on a regular and continuous basis with a profit motive, that's inventory that goes on Sch C instead of an investment on Sch D.

For clients who sell the home that they live in, I also report on Sch D even if not required.  Property values around here are so high that there's almost guaranteed to be a 1099-S out there somewhere.  The last property I sold had it buried in the closing docs.  It takes less time to report it and exclude the gain than it does to convince the taxpayer that it exists and they need to find it.