I'm still not sure I'm understanding the question, but I'll give it a try.
If you don't originate the loan, but simply buy it on the secondary market, either with or without servicing, then I do not believe those relationships are subject to the CIP rules. The rules exempt accounts acquired through the purchase of assets.
As far as OFAC goes, that's an interesting question I hadn't really thought of. OFAC has an extremely broad reach, so it is conceivable that buying a loan from another entity that was made to someone on the list would be a violation of the OFAC rules. The lender who made the loan certainly had an obligation to check OFAC, and if they made a loan to someone on the list, they did violate OFAC. I'm just not sure that your purchase of the loan also violates OFAC. Anyone else out there checking loans serviced by others against OFAC before you buy them?
Also, for what it's worth, if the only relationship my bank had with a customer was a loan we owned, but had not originated and did not service, I would not consider them an existing customer under our CIP program. I think that's a judgment call, and obviously should be based on whether you have a reasonable basis to believe you know the customer's true identity.
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Opinions are mine and not necessarily my employer's.