HOw to get a seller to carry back 20-30% as a second

[Ty MacArthur]

Hey BP,

Has anyone had any luck asking a seller to carry back a 2nd for say 20-30% of the Purchase price? How would you go about asking something like this? I'd like to put as little down as possible. 

Here is the deal I'm looking at.

4 units in California beach town. Asking 999k.

duplexes are on separate lots and I believe I could sell on lot for around 600k. I'd like to hold the other and turn it into a vacation rental or just rent it out. 

Thanks!