Hi Sid,
Its late here so bare with me, but lets walk through the two.
Scenario 1- You will need an option agreement. Basically you say to the seller look I like the property but I'm not really sure if I can sell this. I want to put an option to buy on your home, and see what I come up with. That gives you time to find the end buyer. However, many buyers are not crazy about locking up their property without cash, so you can offer an option with the first right of refusal. Same thing but if the buyer finds a buyer it gives you the right to buy it or let them sell it. As far as selling it to an RE agency, um no. Realtors will list your home, but are not in the business of buying your home. Now some Realtors do buy, but I would not count on that. You need to find cash buyers first, and just ask what they want and go find those deals... Here in Indy I call my buyers every week and ask what they are looking for. Then I focus on finding those deals...
Scenario 2-Just do a double close. Then you can collect (via close) the "$200k" from your buyer and then close with the seller for "$150k". You just have to have a title company that will play ball and accommodate this type of closing, and your buyer will have to have cash, Hard-money, portfolio loan, or if its a private seller just assign the deal and skip the double closing.
So:
Find Your Buyers/What do they want
Find a title company that will play ball
market and find deals
put deals under contract
sell(double close) or assign deals
collect money
repeat!
Now above is a general overview, but just take it one step at a time and don't let the process over-load you...
Good Luck!