This study is concerned with decentralized trade caused by the informal characterizations of market structure, especially in informal markets of developing countries. The informal markets have their own characterizations of market structure which are different from the ones in the competitive market or the monopoly market. A black market, where trading is illegal but goods are legal, is a good example of an informal market. The illegality forces a seller and a buyer to make a direct contact which results in a bilateral monopoly type of trading. With complete information, the bargaining process leads to the unique black market equilibrium prices. Because of the bargaining process, the detection risk becomes the cost per approach of the seller and it discourages a seller to stay longer in a black market. An increase in the detection risk and the availability of the official goods can reduce the equilibrium prices in a black market, while an increase in the controlled price can increase the equilibrium prices. Thus a government can control the prices in a black market through these variables. When the official market payoffs are uncertain for a buyer and a buyer can return to a black market, there exists the mixed strategy equilibrium. When the initial beliefs close to one, a buyer must randomize between accepting and rejecting the seller's offer. Since the agreement reaches immediately under the complete information assumption, a crucial dimension of real bargaining problem is missed. To solve this problem, the incomplete information model is introduced. With the monotonicity restriction, for a range of initial beliefs of a seller, the refined sequential equilibrium price in a black market is unique. For the initial beliefs close to one, there exists an equilibrium in mixed strategies. With many buyers and sellers in a black market, for a range of seller's initial beliefs, there exists a pooling and separating sequential equilibrium. With the initial beliefs close to one, there exists an equilibrium in mixed strategies. When the quality of products is concerned, the uncertainty in input markets and the uncertainty in the production process, which always exist in the informal markets of developing countries, are the cause of the low quality problem. The free-entry condition cannot improve the quality of products. To solve this problem, a government may have to intervene by imposing quality control on input markets. Since there are small number of input suppliers, quality control on input quality can be successful.