If you have a business of flipping houses, it goes on Schedule C. If you are worried about the IRS looking for it on Schedule D, you can either (a) reply to the IRS notice that it is reported on Schedule C, or (b) ALSO report it on Schedule D, but have an equal amount for the cost and the sale price (resulting in a $0 gain/loss, because the actual transaction is on Schedule C).
If this is an isolated 'flip' that is not something that is regular, it goes on Schedule D.
