Because the new coins issued in ancient rome were worth less than the older coins merchants demanded more new coins for the same product. what is this principle called?

This principle is known as Gresham's Law, which states that "bad money drives out good." When a government issues currency that is perceived to have less intrinsic value, people will tend to spend that less valuable currency while hoarding the more valuable older coins. As a result, the new coins circulate more freely, while the older, more valuable coins are kept out of circulation.