What is your recourse regarding a letter you received from Merchants Credit Guide Co informing you that you had a credit card balance due from 13 years ago when the account was closed?

Write a letter requesting verification and validation of the debt (proof that you owe this debt, that you are liable for paying it, that it is legitimate). Ask when the account was last allegedly used by you. 13 years is far past the reporting time period for this account to show on your credit report and way beyond statute of limitations to be sued to recover the money (unless the debt results from child support obligations, unpaid tax liens or federaly guaranteed student loans). Your questions would most likely alert this company that you are aware their dunn has no "teeth". While it is true that if you have not paid this debt, you still owe it; there is little they can do to collect from you (other than send a letter).