N.C. Gen. Stat. § 45-21.36

When any sale of real estate has been made by a mortgagee, trustee, or other person authorized to make the same, at which the mortgagee, payee or other holder of the obligation thereby secured becomes the purchaser and takes title either directly or indirectly, and thereafter such mortgagee, payee or other holder of the secured obligation, as aforesaid, shall sue for and undertake to recover a deficiency judgment against the mortgagor, trustor or other maker of any such obligation whose property has been so purchased, it shall be competent and lawful for the defendant against whom such deficiency judgment is sought to allege and show as matter of defense and offset, but not by way of counterclaim, that the property sold was fairly worth the amount of the debt secured by it at the time and place of sale or that the amount bid was substantially less than its true value, and, upon such showing, to defeat or offset any deficiency judgment against him, either in whole or in part: Provided, this section shall not affect nor apply to the rights of other purchasers or of innocent third parties, nor shall it be held to affect or defeat the negotiability of any note, bond or other obligation secured by such mortgage, deed of trust or other instrument: Provided, further, this section shall not apply to foreclosure sales made pursuant to an order or decree of court nor to any judgment sought or rendered in any foreclosure suit nor to any sale made and confirmed prior to April 18, 1933.