There can be some great deals new (that typically include install for a reasonable fee) and also I've found some awesome deals on FB marketplace.  

And out of those, most are over $400k, in bad areas, need updating, aren't 'real' duplexes or are 70 years old...sometimes all of those things are true on a property.

One of the reasons I have been successful in rehabs is because I have my projects lined up before I even make the purchase and then the work typically starts the day I close on the property.

What tools or methods should I employ to evaluate its true market value?

This is just one of the many reason property managers are great to have for your investments, I'm one so a little bias on this last part.  

For whatever reason, he doesn't want a realtor.

For many different reasons, MHP's and RVP's are the direction I am going. 

Additionally, I am aware that once I move out, I can list the home on VRBO and several other platforms to help boost bookings.The reason why I'm considering/leaning toward selling the house, is that when I begin to run the numbers, the expenses exceed what I could potentially make.

I haven't found any information about not being able to split the lot if the house is built before 1978 (which mine is) but I'll bring that up to the county when I meet with them in a couple weeks.Is there a reason why you chose to split the lot as opposed to adding an SB9 unit and then adding an ADU to it?

the reason is my cousin who I am trustee for owns a rental property.