Adding to what Michael and Nurca said, You have to know how quickly the names will sell even before you buy it. Do not determine how quickly you can sell it after you buy. It may or may not work, but most likely you will NOT get optimum value for it.
When I buy some names, I have reserved a 3-5 year time line for it's sale. Some names, I have appropriated a 1 year hold time. Then again, I happen to buy a name which I know before hand it will sell in a week, no matter what.
Just my 1 cent.