Specifically, I am looking at partnering with an individual who owns securities (stock and mutual funds) having them take out a line of credit against their portfolio (SBLOC) at a low interest rate, and using that to purchase properties rather than using a mortgage.

If the change has occurred and buyers are now doing this, surprised not discussed more  You as the buyer is paying for the agents commission one way or another , either by a higher sales price , or by cutting a check to your agent . 

Cable, smart TV's and guests sign into there own accounts

Typically when you see "cash only" the agent doesn't believe the property can pass a home inspection which isn't relevant for construction loans in most instances and there are plenty of banks, hard money lenders etc. who offer construction financing on distressed buildings.

The owners at the time were directed to our Real Estate agent and she showed us the property as it was a good investment opportunity.How did you finance this deal?

Tax accountants, investor friendly agents, 1031 exhange experts etc.

I live in Phx but we do most of our investing out of state, so I’m not also the most current on local trends: so find yourself a good local agent to confirm that:- it’s going to be hard to find anything at < $150k / door.

The general idea is that you must acquire an interest in real property you do not already own, and since you already own the subject real property you want to build on, the transaction must be structured so that you are creating an interest in real property you do not already own (in fact, it does not yet exist).

You may be better off trying to connect with some agents or other investors/wholesalers in your area and try and get off-market leads sent to you.

If you have a good agent at Networth Realty they will get you a fair deal and keep both sides honest.