Hi @Noah Hoffman, and welcome to BP.That 20%-25% equity can come from multiple places:"Natural" appreciation, which is simply tied to supply/demand for housing and the local economy.

I've recently cut ties with my equity partner (50/50) in flipping houses and want to do it on my own.

As previously stated, I think you are wise to sign a buyer agency but one that does not tie your hands.But where I strongly disagree with his comments is to suggest you call the listing agent. 

They tied it off in the attic, cut out a huge section and replaced it with PVC.

Or if they seem tied on the financial side and had equal ability to pay, I would also make sure to check how they did on following rules, excessive wear and tear, noise, unauthorized occupants/pets, etc.

Then I would tie it up for as much below that price as possible.

Comps is critical and when you have an appraiser and/or realtor not wanting to put in more efforts to find them, lender’s hands are then tied.

Cavity walls are tied to the STRUCTURE using what are called wall ties, and they have very limited structural properties. 

Having been in their shoes, I did everything I could to be considerate of their work flow.  

Tying it all togetherThis year I will be more focused than ever on both my traditional financial investments and the investments I have mentioned here.