I once heard that you can sell a blessed item, but if you do, it is no longer blessed. Is this true?
A blessed item may be sold but only for its intrinsic value – not the value of its blessing (which cannot be ascertained anyway). The item remains blessed.
The sale of a spiritual good (such as a blessing) is called simony which is an offense against the first commandment. Here is what the Catechism of the Catholic Church (2121) teaches:
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.