American Greetings Corp., which sells digital greeting cards and related products online, has reached a deal to be taken private by a buyout group owned by the Weiss family, which dominates top management of the company. The deal is valued at $878 million, including the e-retailer’s debt obligations. The Weiss family includes the company’s chairman, Morry Weiss, as well as his sons, Zev, the chief executive, and Jeffrey, the president and chief operating officer.