The most important thing that a wholesaler can do to build these relationships is to make personal contacts and provide the services that are needed by each side.
The wholesaler in this case should start by getting to know both farmers and retailers to some degree. This will create a comfort level for all parties in the transaction. The wholesaler must then determine what needs each side has and try to fill those needs. Finally, the wholesaler might need to create demand among the buyers for the product. In this particular case, it might be useful to sell retailers on the idea that these dairy products can be good sellers. The retailers might be a bit reticent to buy such things since they would probably be more expensive than more generic dairy products would be. The wholesaler should persuade the retailer that these products can actually make money as premium products.