Favorite amortization schedule for seller-financing

[Pat Jackson]

I've heard some say the rule of thumb is "short as possible".  Others just go out to 30 years.  Still others figure out a realistic monthly payment, work back from there, and have a unique schedule like 21.75 years (261 months).  

I'm wanting to get into seller-financing where the focus is the monthly payment, not the purchase price.  The average American is in a house 7-10 years, then either sells or refinances.  Using this average, is it anywhere close to a safe assumption to amortize over 30 years, think the buyer will either pay or things will go sideways sooner than later?

Another way to ask this, "Is there any reason to not amortize over 30 years for seller-finance deals".  These houses market value will be 50-100k, fyi.