Is it correct that if a creditor purchases auto loans from a car dealer, the creditor would be bound to comply with Reg B as long as it owns the loans?

Yes.

When you buy loans, you buy the good and the bad. But also, the way this was worded sounds like Reg B compliance stops when the loan is made. There are ways to discriminate on existing accounts so the better answer may be yes you have some liability based on the dealers actions, and yes Reg B applies to the way you service the account today and tomorrow.