Effective December 15, if a seller does not accept a “buy-in” notice, the seller must send a signed, written rejection to the buyer no later than 6:00 PM, Eastern Standard Time. If no rejection is sent, the notice is accepted. A “buy-in” notice is sent to a seller when the seller fails to deliver securities. It allows the buyer to repurchase the securities, with the seller making up any price difference. Relevant to: Series 7, Series 24, Series 62.
http://www.finra.org/Industry/Regulation/Notices/2010/P122271