I love the 250k  500k I suspect though with what sounds like 150k and under houses this might not really play out..

The obvious play would be for the low purchase price/cash flow. 

There are other factors at play, but this was just the first thought.

It's sell, take the 100k profit, your at 360k in the bank, VTSAX or money market and chill for a bit, see how this market/economy thing plays out, rates have to come down no one can afford at these rates, then maybe look to refi and get the rate under 5%.Also something to think about.....

a) everyone can use a pool, b) not everyone can or will use a pickleball court c) almost everyone who is a bit older who started playing pickleball ended up injuring themselves, d) a pool is a lot prettier and adds a lot of ambiance to a backyard.

All you'd need at that point would be RE attorneys for the contract and formalities.To find recommendations on seller financing deal structure, I'd start right here on BiggerPockets by using the SEARCH feature and playing around with different keywords i.e. owner financing, seller finance deal structure, seller finance terms, owner finance terms, etc.All the best!

If he's getting 10-12% return, why not borrow at 8% and play the spread?

That's probably not a pure parking play though.   

Would this be a slum lord play?

You may have a grasp on this aspect, but there are items to consider that can play a big factor when choosing your land such as topography, existing zoning & platted lot status, minimum/maximum requirements such as parking, detention, size of building, setbacks.