@John Ellis - You will likely not be able to "cut out" the costs of selling your flip. While some get their license and avoid the list agent fee, some use flat rate listing services, you will not get around having to pay the buyer's agent fee (if you place on the MLS which is necessary).
Now, on to lining up buyers before you have to list. This is done in many ways. When you are in the middle of a rehab, you can place a yard for sale sign with your contact info and take any and all calls from potential buyers that call on your sign. You can direct market to people. You can place internet adds to attract buyers. You can ask everyone you know for referrals of anyone looking to buy a home. The list goes on. I have sold a few properties before I was done with the rehab, however, in each case, I still paid a buyers agent. With that said, what I saved was holding time saving costs and thus, increasing my profit margin.
If your deals are so tight that you need to save the 5% on agent fees, you do not have good enough spreads. If you are simply wanting to increase your spreads even more by eliminating these costs, I would say that you would be better served as a RE investor by getting more deals and more funding for your deals rather than concentrating on saving RE commissions.