How do people flip houses already listed?

This seems impossible; I go to and see alot of potentially good properties to flip (esp ones selling for 7-9k) but then I realize that if I put a contract on these houses there is a good chance my potential investors I will be marketing this house to have already seen and passed. Why in the world would they buy it at a marked up rate.

In an experienced “flipper’s” opinion do you stay completely away from listed houses (foreclosure websites, mls, homes magazines)? I would assume the only potential for a flip is to bring value to the table for investors willing to buy by finding sellers before they list. Should I save the MLS and foreclosure websites for buy/rent cash flow opps only?