Since you are a CA resident, I would also recommend checking out the Delaware Statutory Trust as an option for entity structuring - its a good model for CA residents as it avoid the franchise tax and is scalable if you purchase additional properties.Thanks,-Megan

Don't worry about size, its all scalable. 

I am curious to hear from real estate agents and flipper which business model is sustainable and scalable in the long run as both these professions are considered active income.

In my opinion, it's better to put scalable systems in place on the front end, than to change things midstream when you outgrow something that was too basic. 

In the upstream oil business we would routinely buy mom and pop small businesses for a multiple of EBITDA based on the stability, scalability, and transferability of the revenue.

This solution is not scalable once we have large number of units. how other people are doing here?

My initial thought was to network with several wholesellers to enlist their services and leverage their scalability of searching, marketing, and purchasing.  

One of the most difficult aspects of flipping houses is the scalability.  

I'm not a huge fan of condo investments myself (your investment is at the health and whim of an HOA) but maybe that's a more scalable option for you.

Changing filters yourself is not scalable.