Buying an existing business is usually a better idea than starting something up. Be careful that they’re selling for the right reasons (i.e. retiring, health, moving, etc.). Check out http://www.bizben.com for example. Look for things that are not real estate or not discretionary income related – may not leave much, but worth looking there. A lot of times you can buy an existing business that will cash flow on day one for not much more than it would cost you to set it up yourself.
Be sure to get professional legal and accounting advice, and to haggle!
