Everyone has a love hate relationship with brokers. They buy your stuff for a better price. They sometimes have what you want for a better deal than the AH.
Many times, they can have nothing at all.
I think instead of just purging their contents every so often, they should hike their butts a few yards away and offload their wares to the AH for sale at their usual markup for the BuyNow option. All proceeds go to the loot tables.
In game it could look exactly like that. The broker wanders to the AH for a bit, and the list is loaded up to the AH with base value bid and sell markup as buy now.
This would get the items off the broker list and help force the AH economy for random loot to stabalize.