You need a license to advertise a property, no option can give you the right to advertise or show a property and not be considered a listing agreement, such agreements may cause the option to be an installment sale as well. 

Should I install a sprinkler system or let it be? 

However, like an RV a Park Model cannot be permitted as an "Additional Dwelling Unit" (ADU) aka "Granny Flat" and therefore is not allowed to be permitted or permanently installed on a real estate lot or parcel.Unlike their big brother the HUD Manufactured Home, Park Models are built under the ANSI code and cannot exceed 400 square feet of living space.

Typically we just make sure the grey box is install properly but I personally have not had them installed or requested to be doing bank work or flips.

What is needed to complete:-Additional painting and touchup painting of walls, trim, and blinds-Repairs to oak wood flooring, plus sanding, sealing, and clear-coating the entire floor-Laying marble tile around a fireplace-Installing a sink in a granite countertop, including cutting the holes, etc.

What about people like an AC company who installed a new AC?

He has came to me and said they have money to make alterations to the house that would include installing a backup generator, ramp, shower, and a few other items.

I wouldn't go to the trouble of an installment contract for 1 month and legal title isn't conveyed.

In the early days, they did all the rehabbing THEMSELVES (installing new dishwashers, etc.) 

Wherever there's insulation installed, there's always a moisture barrier with it (if it's done per codes).