Any conventional loan lender will ask for the same documentation.

If you don't then make sure you document everything if you do anything other than the regular payment.

Of course, that's why you should document everything and send your required notices.

Now this is a small six unit building, with ALL previous owners gone and new owners either moved in or rented out, two of the six units are in foreclosure and hence vacant, and the new owners are getting familiar with rules and regulations of the HOA documents which are like six phone books, so the questions about who's responsible is met by "...well I don't know either what do YOU think?"

But, here is some advice that you can use in the future.Never accept a deposit to hold something without documentation on keeping it if they back out.

You would need to see the actual note document to be sure about any particular loan, but the general terms are standarized:http://www.law.cornell.edu/uscode/text/12/1701j-3(d)Exemption of specified transfers or dispositionsWith respect to a real property loan secured by a lien on residential real property containing less than five dwelling units, including a lien on the stock allocated to a dwelling unit in a cooperative housing corporation, or on a residential manufactured home, a lender may not exercise its option pursuant to a due-on-sale clause upon—...(4) the granting of a leasehold interest of three years or less not containing an option to purchase;...This section gives a exception to the due on sale clause for a normal lease as long as it three years or less.

Best to speak with an attorney to get the documentation correct and be sure you're in compliance with any relevant laws.

All that should be formalized in the documents for the house.

(Electric, lets say) What document is used to record the sale of the lien?

Before I get into any deal, I want to find a lawyer who can prepare a contract and some necessary documents to mitigate potential risk involved in subject to transaction.