Can't really put my finger on it but smells sort of like body odor, cigarette smoke and uncleaned clothes.

25% for flood when creek rises, fire, non paying tenant, roof repairs, tree falls on duplex and insurance deductible is $2500, tenant puts toy down toilet and clogs main line, dog which you did not allow bites someone, disposal stops working, property manager 10%, property taxes increase because they see you did rehab or you changed the title, noise fine as tenant decides to become a drummer, trash bill, ants or vermin and you have to hire a exterminator, mold claim, planning department inspection fail, city fee as business, legal fees, CPA fees, insurance goes up, Imperial Beach business license and landlord permit/ annual fire safety inspection/gas co inspection $1500+, heaven forbit the Coastal Commission says you dumped foul water, to name a few

Took 6 months and we had to LITERALLY use respirators to walk inside the house because the odor / fumes were so bad from the cats...

I'll track down a company who can identify the odor and perform the cleaning and get itemized receipts. 

I agree with everything above, and let me add this.Their first child is born in that living room = +$30KThey build this UGLY raingod totem in the backyard = +$15KThey added $315,375.05 in "upgrades" in a $104,466.03 average home value area and thus = +$400K15 years ago, they bought the house for $400K and thus the house is worth X + $400KLand = $X, House = $Y thus everything is worth $X+$YFSBO (Florida Seniors with Bad Odor) are often uneducated on the market.

I will say, most cabins are on well water, which can be tricky - the water can be very mineral-y and, depending on the well and conditions, can have an odor (sulfur) or some color.  

No harm no foul?

In my mind its our inheritance, and I'd rather not foul things up, and be required to give the government a sizable portion of it, if that can legally be avoided.   

Plus the washer gasket (although replaceable) is a mold/odor haven that is very nasty to swap out & everyone complains about it.

Never been faced with a huge odor issue but from what I've read on other posts, the ozone machine is the way to go after a good clean