The other owners would sell you their shares for the value you agree on, and the loan funding (your new debt) would be their proceeds.

I have found a lot of success in the past and not so far past in purchasing lots, building an "affordable" home and selling it as a spec home. 

So they are selling 1/2 a duplex and someone owns the other half?

That's going to be almost entirely owner occupied stuff where single family houses are selling for above $250k and things of that nature. 

So someone toes up a property via a purchase contract and then sells that contract to a third party.  

Flipper is going to sell the houses to the highest bidder.

I'd rent in Jacksonville for a minute and keep your current and run it as an STR vs. selling.

The listing has been live for 180 days and has yet to sell through my real estate agents.

Because I have another investor on this project and I have to pay him out of the profit, and also is it possible to get a pre approval letter before selling my investment property because it throws off my debt to income ratio, but it's an investment property that is meant to be sold.

As rates drop further and the spring/summer selling season will push values higher.