Why is johnny tremain foolish to try to sell the cup to Mr. Lyte?

Jonny is pretty much homeless at this point. He really needs money. He tries to sell the silver cup for cash, but the local silversmiths will only offer him four pounds for it because the design is so old-fashioned. He knows it is worth more to Mr. Lyte because it is part of a set, so he goes back to the Lyte house and offers to sell it. Johnny needed to sell the cup in order to survive.