Final answer:
Carmax's promise to not revoke its offer without a stated time period creates an open-ended commitment, implying that the offer is valid until further notice.
Step-by-step explanation:
When Carmax promises not to revoke its offer without stating a time period, it creates an open-ended commitment. This means that Carmax is guaranteeing not to withdraw the offer indefinitely, until otherwise stated. However, without a specific time frame, the offer could theoretically be revoked at any point in the future.