Use some leverage, but only to what you have comming in!
I personally like to purchase at those "free and clear" tax sales. I do find that building wealth "free and clear" is the only way to weather the storm (economically) these days.
While everyone else was screaming about their lines of credit being reudced, I went into my bank with my "free and clear" deed and obtained a line of credit.
Using that line of credit of 36K I now have increased the equity of the house from 58K to 125K-36K which is 89K.
But doing this 5 different times all at once would bring my total payments to those lines of credit to a level higher that I could make if I do not have someone paying me for the house (rent) and to sell one I would have to pay off that 36K which is like asking the buyer to put almost 30% down.
The point, I guess, that I am trying to make is some leverage is ok but too much (leverage as a business model) will bite you in the end one day.
I think free and clear is much better. But then that takes one to "How do you buy those flips?" Since cash is king you can often get a greater discount paying full cash right now!!
And the lower you leverage that property the lower you can accept as downpayment when selling using owner financing, which I think is the way to go as now not only do you (acting as bank) get all the equity by selling at your price, but you also get to have the interest that would usually go to the banks.
This alone will greatly build your wealth.