Give your female cow mirical potion. Once you do that, wait until she gives birth. After that, milk the female cow twice a day. Once in the morning and once in the afternoon (do this every day within the 40 days after she gives birth). Then when Van comes, go talk to him. Say you want to sell something. Give him the milk. He'll tell you how much one bottle of milk cost.

Put in the number of bottles of milk you want to sell(should be more than 20). He'll give you a deal. Don't take it. Then he'll give you another deal for it. Don't take it. Finally he'll give the last offer. Take it. As you will notice, the amount of gold increases each deal. I did this with 67 bottles of milk. I got at least 10,000gold