Buyers list first. @Detra Sheppard is right. You're going to find that it's going to take you a bit to find out who on that buyers list is a true buyer. Depending on your method for building a buyers list you may find out of 10 prospective buyers you only get a good solid response from 2 or 3. If you want to get even more detailed about your buyers... all buyers are not the same. For instance contractor buyers will pay more for properties because they don't have to hire a general contractor vs an investor who does have to pay GC costs.
There are some buyers that will buy anywhere and some that just want to buy in certain areas. Why would anyone want to wait until they have a property under contract only to find out that the few buyers they just contacted don't like the area your newly found wholesale property is in?
Why would anyone want to wait until they have a property under contract only to find out that the few buyers they just contacted are tapped out and need to sell properties they are currently rehabbing before they can buy your deal?
Some say that if you get a property under contract at the right price there will always be a buyer. That may be true but It just doesn't make sense to me. Once you start the clock on your contract you now have to use 1- 3 weeks of that contract time looking for a buyer?
IMO the same day you find a wholesale deal is the perfect day to shop it to your cash buyers.