Simply, a stock purchase agreement is a contract between a privately held firm and the buyer to transfer the firm's shares. The agreement describes different conditions, such as. OTC products are usually for longer maturities, and are usually a form of options product. For example, the right but not the obligation to cash delivery based on the difference between the designated strike price, and the value of the designated index at the expiration date. These are traded in the wholesale market, but are often used as the basis of guaranteed equity products, which offer retail buyers a participation if the equity index rises over time, but which provides guaranteed return of capital if the index falls. Sometimes these products can take the form of exotic options (for example Asian options or Quanto options). The demand is the number of shares investors wish to buy at exactly that same time. The price of the stock moves in order to achieve and maintain equilibrium. You're selling personal property and want a written agreement. You're buying property and want written proof of the agreement.You want assurance that the seller has the legal right to sell the property.As the seller, you want to document payment agreements. The Shareholders and the Corporation desire to make provisions for future disposition of the Shares in order to prevent interference with the orderly conduct of the business of the Corporation.