Hey David, this sounds like a great project with the potential to add a lot of value to your property.

It seems you are the only person who understood the question wanting to know what a seller makes (not even sure how or why you think that).As a typical part of the DD process it is not uncommon to ask for property/entity tax returns.

Property Sellwise

They commented that they were indeed interested in selling the park.

@Moshe Cohen   The easy answer is called "delayed financing" assuming you are buying/bought the property with cash. 

How is general property appreciation there?

I am willing to offer 5% ownership of any properties I purchase with them as co-signers.

If they've owned the property for 15yrs, probably a better chance of owner fatigue than someone only having it for 2 yrs.

I recently bought two vacant lots in Canton, Mississippi, in Madison County, and I plan to sell them in the near future. 

There are different types of investments like residential properties, commercial real estate, or short-term rentals.