I was actually just made aware today of a house that has really good potential for "owner financing". I assume the idea with those is we agree that house is currently worth $X and anything over $X minus renovations is split between me and owner? Are there boiler plate contracts for this type of arrangement, what stops them from screwing me after the sale and not giving me my share?
That's not a common arrangement, but could work.
It's all negotiable, basically.
Easiest thing to do, IMO, would just be to buy it from them on owner financing, where they write the mortgage for it to you (they're acting as the bank). You disclose everything up front, that you will be flipping it. Set some easy terms, like $0 down and no payments for the first year (you'll pay $1 now for the property, some consideration). They get a price they are happy with (perhaps even more than they were expecting), have no risk as you will be paying a bunch of money to fix up their property (which they can take back as the lender if you default), you get a house for nothing down, can use partner money to fix it up, and then pay for the house when you sell it.
You don't have to worry about them screwing you, as the property is titled in your name. You own it. You get the money when it's sold. They don't have to worry about you screwing them, as they have a lien on the property (same as a bank with a mortgage would) that has to be paid off when it's sold.
There's a million ways to structure it. That's only one. The key is to learn about as many as possible, so you can choose the one that is most appropriate for the situation. Find out what the owner needs. (Often it may just be a small amount for moving costs.) Then use the appropriate tool to help them and create a win-win.