South Africa, Dec. 20 -- As Christmas shopping ramps up, you may be getting a lot of emails offering you attractive discounts for a short period only. You may see flash sales or special deals that exhort you to "buy now" to avoid missing out.

These digital "time-limited" offers, as they are called, are actually an old sales tactic.

Those in the game of selling cars, for example, have long used the trick of alluding to that other very interested buyer who's likely to return and snap up the bargain that's before you. Telephone salespeople routinely offer deals that must be accepted during the call. Want time to think about it? Too bad.

Online time-limited sales work on the same basis, but with technology taking it to a whole new level. N...