When does buy, rent, and then sell trigger self employment taxes?

[Brian V.]

It is my understanding that if I buy a property and rent it out, that I do not owe self employment taxes. Now lets say that in 1 year, I sell the house and buy another one (or two) and rent it out for a year. The income coming from the rentals are passive, and the sale of the house for a gain is inevitable.

At what point do the activities of buying, renting, and eventually selling after a year or two trigger self employment tax. I have read that if I am a flipper that my income is subject to self employment tax. But if I'm buying and holding, does that trigger the self employment tax? At what point do my real estate activities become subject to this type of tax--about 15% for  social security and medicare.