Biggest Issues in Real Estate Investing, Stupid Questions or not?

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What is the biggest issue or most important thing for your business with real estate investing?

#1. Finding good deals... meaning houses that can be flipped?
#2. Flipping the houses at the cheapest possible costs?

#3. Or selling the houses after they have been flipped?

... Not sure which one is biggest issue or most important but like how money / cash flow makes a business an actual business then i would probably guess that selling a house quickly after it has been flipped would be the ideal situation.