Remember you only make money when you BUY a property, not when you sell it. That means that you have to "make" your money when you buy the house. Make sure you have enough equity in the house upfront to:
1. Repair the house for sale. This includes all remodeling, landscaping, etc.
2. Carry the costs of ownership of the house while the repairs are happening. If you take 45 days to repair the house, make sure you have 2 months worth of mortgage payments onhand. This is called "carrying costs"
3. Pay the real estate agents. Typically you will need to pay between 4-6% of your selling price in commissions to sell the house, depending on your area. If you don't use a realtor, you can save that fee, but you might not get your property sold in a timely fashion and may eventually get less money for the house.
4. Cover the capital gains tax. Remember, short term flips are taxed at 25% federal and (in CA) 9% state. Make sure you get sound tax advice before you spend your profits.
A good rule of thumb is to find out what the comps are like in the area, offer 70-80% of the comp value (this is CA, by the way .. if you aren't in a "hot" market, like CA, HI, FL, NY, etc, then offer 50-60%). This should give you enough room to carry the loan, rehab the house and, to quote The Steve Miller Band, "Take the money and run!"