Jackson offers to pay Casey $50 for his used business law textbook. Casey agrees to sell it for that amount. They agree to meet one week later to exchange the money for the book. Casey and Jackson have formed?

a valid contract

Explanation:

Since both Jackson and Casey freely engaged into a valid and binding agreement (Jackson made an offer that Casey accepted), and there is consideration involved (both parties are exchanging something of value: money for a book) then a valid contract has been formed.