During the first year of Chapter Two, Van will sell goats during Spring. They initially cost 4000G each, but can be bargained down to a price of 2000G.
Unfortunately, your goat can't be sold in Harvest Moon: A Wonderful Life. In order to make more room for other animals in your barn, it may be necessary to intentionally allow the goat to die. You can sell your goat to Van in Harvest Moon: A Wonderful Life Special Edition and Harvest Moon: Another Wonderful Life.