If they're only willing to pay by check, it could be a red flag in my opinion. 

Get out of your comfort zone.

Do you go to conferences, yes, but when I see their ads for their courses, books etc. to me that is a HUGE red flag.

I assume zoned agricultural.  

Likely built before the area was zoned and this is a non-issue, it's a non-conforming property grandfathered, if the zoning is single unit single family.

She would have money for Aug rent, if not red flags because what was she going to do if she stayed.Her leaving on the 20th means you have some time to go in and clean it up (probably not much to do as she was there for 4 months) and maybe get it rented by Aug 15.

The property does not have a basement or sump pump and is not in a flood zone.

Talking to zoning officers, I find that in some municipalities they are grandfathered in, but in others this exemption ends when the lot changes hands.What I am seeing is a picture of a huge amount of semi-rural land that is frozen in time—impossible to use and therefore impossible to sell.

Zoning meeting this Tuesday has ADU ammendment on the agenda.

If so that can work in very specific red hot markets.. however experience has dictated to me over the decades that land can take a long time to sell .. many times has to be sold on contract to move it.. or if its cash you have to have the lowest price parcel of any similar ones on the market to move it timely for cash..