While that may sometime be true, full disclosure to all parties should eliminate any ethical or moral concerns (imo).  

As far as benefit to the seller goes, @Matthew Paul I think you are probably right on the money and regardless of whether they would have preferred to stay in this house rather than move, it is likely providing them some excellent cash flow; and I'd be hard-pressed to come up with a deal that serves both parties better than the current situation. 

The relinquished property was owned by one taxpayer and the replacement property was owned by a different taxpayer, although related parties or entities.

Work with a real estate attorney to draft a formal agreement that outlines responsibilities and protects both parties. 

There is a way to do it with respect to all parties, but that's not why people are signing up.

Challenge: It's in a neighborhood (no HOA), and renting the entire property (sleeps 25 in total) caused problems in the neighborhood, due to noise and parties – so we only rent the separate cabins now.

Work with your commercial lender/insurer/lawyer/personal banker and see if they know of any interested parties.4. 

I have seen a number of cases locally where a party clearly did something illegal like this whether it be a landlord tenant or someone hiring a contractor.

Don't solely rely on third-party management companies; instead, focus on building a solid asset management plan and, if needed, bring in experts or partners who excel in this field.

It is common practice for mortgage companies to later sell the already funded loan to a 3rd party like a hedge fund or a bank after the fact.