Short holds in SDIRA and what is considered a "business"?

[Chris Martin]

It looks like "flips" in a SDIRA are subject to UBIT. But rental income is not subject to UBIT. What about gains on the sale of rental property? From http://www.nuwireinvestor.com/howtos/how-to-keep-your-self-directed-ira-healthy-and-the-irs-51470.aspx

"The government requires that all businesses in the U.S. pay taxes. If any business sells some or all of its products or services without paying taxes, it would have an unfair advantage over its competitors. For this reason, UBIT could be triggered if your IRA owns all or part of a business that earns its income through the sale of a product or service. Flipping real estate and developing land for resale are examples of investments that are, more often than not, considered "businesses"—which makes profits generated by such activities fair game for taxation." David Nilssen, co-founder and CEO of Guidant Financial Group, Inc.

So, if the SDIRA holds a rental property for, say, two years and then sells the property for a gain... is UBIT due on the gain? What about one year? What if the property is sold with seller financing to the tenant after 18 months? 6 months? Does 'intent' play any role in what is considered a "businesses" to the IRS? Any insight appreciated.