Items may be sold; blessings may not. Any blessed item that is sold should be sold for its intrinsic value apart from the blessing. To sell the blessing itself would amount to simony.
The Catechism of the Catholic Church addresses this:
Simony is defined as the buying or selling of spiritual things. To Simon the magician, who wanted to buy the spiritual power he saw at work in the apostles, St. Peter responded: “Your silver perish with you, because you thought you could obtain God’s gift with money!” Peter thus held to the words of Jesus: “You received without pay, give without pay.” It is impossible to appropriate to oneself spiritual goods and behave toward them as their owner or master, for they have their source in God. One can receive them only from him, without payment. (2121)