On the market for 25 days; possible outcome: seller financing , Lease-Option, or sale and purchase more through 1031 exchange.

In your situation I would also think about putting 1031 exchanges to use.

Unless you're asking if the buyer can use the 1031 exchange to purchase a property that they are currently renting and have an option to purchase. 

If you miss your window and haven't lived there 2 of the last 5 years (like if you choose to sell several years down the road) consider a 1031 exchange and you may be able to get out of the capital gains as well.

They would likely have the tax liability immediately if it's not primary residence, so spreading it out over years may be helpful if they're not doing a 1031 exchange into a new investment property. 

Our arrangement called for him to do the vast majority of the work in exchange for 50% of the profit at the end.

So I would appreciate it if you could assist me with some input on how to reach out to professionals and offer my assistance in exchange for experience.I am in the Tampa Bay area.

But can they claim they own the improvement and want some rent credit or payment in exchange at the end of the lease? 

I have some family members that are interested in putting in some money to help cover the down payment in exchange for equity and a preferred return. 

A 1031 exchange is an option but speak to a CPA beforehand.