-My previous car was a Pontiac Grand Prix which had so many problems, some of which mechanics couldn't diagnose.

He diagnosed my problem as a faulty control board.

Wow Bobby - Do you do your own plumbing work or hire an expert to diagnose and fix the problem?

Sure, I can call out a HVAC company to diagnose Aka repair the unit to see what it needs.

Now, I don't know to what lengths a seller is required to diagnose a problem if the buyer doesn't request more info prior to the sale.

Get more diagnoses (2nd opinions) and quotes ASAP. 

My in laws were selling their house about 2 years ago for about 250k but took it off the market due to my wife's step dad being diagnosed with Amyloidosis.

If they don't understand personal financial and have a plan for the future move on.After 23 year's in the military I went from private to sergeant major but was diagnosed with cancer.

Will the manager send an electrician who charges &125-$150 just to diagnose or will the manager send in a handyman who will charge $20 per hour and can help complete basic troubleshooting?

I in the process of removing a wall just had a cad designer do some plans and now the engineer is wants tons of additional specs Look for another engineer, just like doctors, get a second opinion when diagnosing bearing walls. usually you need an upgrade of the columns and beams