When working with private money lenders, what is the best agreement in your opinion with regards to ROI for them? 

It's like a RE on-market transaction...they sign an agreement with you that any property they buy in the next 30 days you're involved in, you get paid. 

Via an operating agreement?

But it works, simplified, like if you owed $500 to CA but paid state XX $300, then you only have to pay CA the $200 difference.CA may not be able to get blood from a turnip, but they will suck that turnip dry before they let it go.Likewise if you, say, play professional sports for a team in state XX, live in state XX, make $20,000,000 a year, play 1 out of 20 games in CA, they want that 1/20th of that two million taxed in CA...but they will give a credit for the taxes you paid in state XX according to the interstate agreement. 

Also, when investing with family & friends, you may wish to use a JV agreement or some other document? 

I have an investor that just signed a purchase agreement with a seller.

We split everything 50/50 with no prior operating agreement, LLC, or paperwork.

We exchanged about 3 sets of offer/counter offers until we came to agreement: I would wholesale his property to a cash buyer with no inspections.

Check over your management agreement and have a conversation with them. 

Would I create an LLC for the property with an operating agreement?