Unless stock is publicly traded, there is no market to set the value of stock. The value is really what the corporation is worth, divided by the number of shares of stock issued. In most small corporations, there is usually no need to determine value unless the parties negotiate a stock sale or a shareholder is leaving the business. The board of directors has the authority to determine stock values, and corporate law provides that the value established by the board of directors will be upheld without fraud. Directors may get an appraisal from an accountant or business appraiser to assist them in determining stock value. Many different methods are used to appraise the value of a business and its stock.
Note: An appraisal is merely an opinion, and several different appraisers may come up with different values.