MODIFY:::Add sell fonctionnality
	modified:   WEB-INF/classes/dao/MarketDAO.java
	modified:   WEB-INF/classes/dao/MarketDAOImpl.java
	modified:   WEB-INF/classes/servlet/BuyServlet.java
	modified:   market.jsp

	TODO: clean MarketDAOImpl.java code, verify user money, get userId