@Michael Zacher
1. I agree with Aaron. However there are instances where I would send the first class letter (not postcard) to the property address and have on the enveloped "Address Correction Requested" or "Forward to Recipient Address." In the first care the letter will come back to you with the new/correct address. In the 2nd case, it will be forwarded. I do this when I cannot fine the mailing address for the owner.
2. Building a buyers' base and vetting potential buyers (by actually talking to them and asking questions) will minimize the risk of not being able to flip the property. In any case, you should always have a contingency clause on your contract, even if you have a buyer lined up.
Some of the questions that we ask to qualify buyers are:
1. What type of property do you buy?
2. How much are you willing to spend?
3. Which areas are your preference?
4. Have you purchased property recently?
5. Which title company do you work with?
6. Are you ready to buy now?