In many cases - properties sell for MORE if there is a vacant unit (especially if it's ready to rent) as you can market the property based off market rents as opposed to lower, inplace actual rents. 

Last 2 years single family has continued to increase due to limited supply even as rates are where they are (which is not actually high).

We've seen this soooo many times in our Detroit market:(Have actually had owners choose not to do business with us because their previous PMC didn't require it - so, they thought we were the incompetent ones.Have also seen incompetent PMCs wiring funds internationally to owners - and these owners not having an ITIN, EIN or SSN!

Most will work hard when they are doing the capital call (a few of my deals did that) but some actually called to discuss the situation.

How many Due on Sale clauses have you actually seen enforced?

Two parents and 4 kids seems reasonable, but my houses are really tight with more than 8 people milling about. 

For the reasons other have said, do your best to keep the rental. 

I have never had an actual deal done.

There are really great REI opportunities in different markets - markets in the Midwest and Southeast of the country are particularly lucrative - where not only are the purchase prices reasonable for most RE investors, but the homes are turnkey (new builds or completely rehabbed homes, tenant ready, systems 10 years of life remaining on them, property management teams in place) with appreciating home value AND appreciating rent.

Fees were reasonable.