old paydayservices.com (illegal) loan with NCA

Back in 2006, I had paid 4 payments of interest to paydayservices.com (ie paydayyes.com) before closing my checking account. They sold my account to National Credit Adjusters before going out of business for being illegal. National Credit Adjusters who bought this **illegal** loan account is still trying to collect $820 from me. I have paid more than the original loan of $300 since paydayservices.com was not licensed to be charging me interest but NCA won't believe me and won't negotiate. Any options??? NCA tells me I'm full of BS and to pay my debts and hangs up on me. Would a debt validation letter work in a case like this since the original creditor is out of business???

Thanks!!