Seems Like A Silly Question
this might seem like a silly question but I have to ask it.
I know how to approach a seller on the phone but what if the seller is another investor?
How would it be different?
i saw an ad and i want to call but I can clearly see that it is an investor. Not quite sure how to approach him. 
Use it as a great learning experience.
Awhile back, when I wanted to create a pool of buyers for my upcoming inventory... I would advertise a fictitious house for sale...and begin building my list of Buyers.
This could be the same thing. So, play along. No need to identify yourself as a pro, or wannabe pro, but just as an average Joe/Josephine looking to buy a home.
Frame your questions as if they came from a novice. See how the pro handles your questions... and try to figure out his game.