The Seller is expecting to get more money.
Seller Financing is preferred by Sellers when the Park will have difficulty getting finance through banks, typically because it will not appraise at their asking price. Really common when there are lots of Park Owned Homes and the Seller uses the home rent component as part of the business value calculation.
Also if the Seller doesn't need a lump sum today and wants a steady stream of income this works, and will also let Seller spread out gains over time.
But based on what you said about this guy it sounds like he structures the deals so that you're setup to lose it. Make sure you have a really good attorney get involved to represent your interests and that terms are reasonable, or walk.
I remember another thread somewhere here about being able to find out who the Seller foreclosed on before so you can find out if it was nicknack stuff or they truly were not qualified owners.