Most of the cybersecurity advice leading into the post-Thanksgiving orgy of shopping known as Black Friday and Cyber Monday has been aimed at consumers. Which is fine – all of us can use the reminders, since criminals are primed to profit from our carelessness or cluelessness about increasingly sophisticated threats in the online shopping world. But, obviously, there are two sides to every transaction. If there is a buyer, there is a seller. And sellers – retailers – could also use some advice.