If we have a participation loan it's usually a percentage that we are buying and the selling bank is responsible for CIP. But what if we buy 100% of the loan? Are we now responsible for CIP?

No. A purchased loan is not an account for CIP.

Pursuant to the CIP rule, an "account" is a formal banking relationship to provide or engage in services, dealings, or other financial transactions, and includes a deposit account, a transaction or asset account, a credit account, or another extension of credit. An account also includes a relationship established to provide a safe deposit box or other safekeeping services or to provide cash management, custodian, or trust services.

An account does not include:

Products or services for which a formal banking relationship is not established with a person, such as check cashing, funds transfer, or the sale of a check or money order.
Any account that the bank acquires. This may include single or multiple accounts as a result of a purchase of assets, acquisition, merger, or assumption of liabilities.
Accounts opened to participate in an employee benefit plan established under the Employee Retirement Income Security Act of 1974.