Who is the collection agency? Is the collection agency attempting to collect the deficiency balance or a judgment? How much money does the collection agency claim you owe?
Information on Avvo should not be construed as legal advice, as each case is different. For information about your specific case, please contact a consumer law attorney, or contact me at www.agrusslawfirm.com
You can, and should, send a letter telling them not to call you or contact you. If you do this, they can send you one more letter saying that they will not contact you. And they can sue you. If they previously obtained a judgment against you, you need to review the case file of the court that entered the judgment. If they have not obtained a judgment, the statute of limitations has expired and you have a good defense.
You should also check your credit report, which you can do for free. See if this account is listed on your credit report. If so, you should have a lawyer dispute the account and then sue the debt collector.
An attorney will be able to access information about your case online. I offer a complimentary service on my website called "Litigation Checkup" which provides information about court filings by large financial institutions against consumers in San Diego County. For more information, visit http://www.jawfirm.com.
If there is not a judgment on file, there may very well be a statute of limitations issue and a possible Fair Debt Collection Practices Act cause of action arising from the debt collector's attempt to collect on a time-barred debt.