It is very rare for a MJ boutique to have a sale... even then it's only a couple bags that are several seasons (or years) old. On the other hand, when they put something on sale the price is amazing! I know several of us PFers were able to get Selmas for $200 each (reg $1100+) back in December.
Marc Jacobs boutiques hold seasonal sales on clothings and shoes. Their bags generally never go on sale; past season items (if still available in stores) are still full price. Stores send items that they no longer keep to some location rather than having sales to clear out their inventories like non-MJ stores.
They really do have great sales, but usually, there's no advance notice, at least barely! We have to rely on girls who have visited the store that day.