I put my signs in the yard and wait for customers to call and give their offers.  

I glanced at Twilio but it seemed to be more geared towards enterprise clients who needed to customize to specific solution.

I would also think good organizational skills, the ability to budget, network, schedule/coordinate, provide good customer service, and a solid knowledge of real estate laws would be essential.Perhaps @Nathan G. could think of some more.

Then maybe buy a lot in a neighborhood that is developed with all the utilities available and just hire a customer to build on it using house plans you buy elsewhere. 

Huge customized closets.

Most come in at around half that, but if windows need to be replaced, or if the exterior doors do and need to be custom sized, it can get expensive quick, so $10k is a safe negotiating number.

But, in my opinion this sounds like an angel investor/customer for you, or somebody really trying to partner with you.

Someone who understood the different rules (DOS/Local MLS rules/State REALTOR rules/national REALTOR rules) as well as customs.

My question is what happens if they back out before closing and what if their taste is too custom.

I will do some looking into customer relationship management (CRM) software and see what I can find.