.* In the absence of similar comps - go back in time to find any similar comps, no matter how long ago and adjust for market trend differences (appreciation, etc), as well as other adjustments as typical (size, bed/bath, condition/quality, location, etc).* Go to nearby market areas/neighborhoods and find reasonably similar duplex sales and adjust for market area/neighborhood differences as required - along with the other adjustments.* Find reasonable 3 or 4 unit (not more than 4, and they can use sfr's, but those are typically not as good for comparison.

If the buyer's strategy is to hold long term, and they are getting favorable interest or payment terms, and the value supports, offering more than ask isn't necessarily a bad thing for them.

How long have they owned the building and what’s the current occupancy?

I plan to refinance this property in a year and a half and turn it into a long-term rental, so I am weighing the short-term benefit advice from this contractor with the risk of getting pegged for doing work without a permit.

Once you have enough cash to sparingly invest in different projects, put some in rentals, use the other to make more.If real estate investing is your long term.

This may be due to my cat long ago left their marks.  

Out of state investor located in Long Island, NY.

How long have you had those two renal properties? 

These are usually legal for an owner to live in (as long as the bathroom is permitted).

If they can't, you could test yourself by driving a long thin nail through the drywall that covers the firewall and see if it hits "something really hard" about 1-1/2" in.