He is preparing a bid/proposal that I presume will show itemized costs for materials and labor.

Google MS Coast REIA for details. 

Are there any books that describe in detail how to become the middle man and set up some rules (e.g. the selling price) even if in person dialogue between both persons (potential buyer, agent) are between them only?

@Mike Dymski provided a solid list of the key items.  

The roof should have at least ten years left for a turnkey.Missing the front gutter on the upper roof.The back yard comes up to the bank parking lot, but it's a bank, so that's probably not a big deal.Based only on the photos, and no walk-through, if the items I mentioned above had been done, then I'd say it'd be worth 40-45k.

Definitely get the details from him.

I don't reside in  Florida(but visit often) so I'd need the assistance of   a credible realtor or  savy investor to iron out the details . 

I picked up one or 2 nice items that I added. 

Additionally, I would add that you should use “classes” for your QB accounting which allows you to assign a class to each property and a class for itemized expenses within each property.

@John Leavelle Thank you for the detailed response and insights, people like you are what make BP great.