How can you prevent buyers from undercutting the wholesaler by trying to negotiate with the seller directly? Should you have them sign an agreement before showing the property?
Once you have a property under contract for a certain price and the contract between you and the seller is binding. The seller cannot sell his property to anyone else. If they fail to comply you can sue.
When you market your property to buyers, list that “assignment of contract ready for purchase” showing that you have the property under contract and ready to sell.