Possible Clues:

• By the sound of it, not the whole of the sale for the trade
• What one doesn't pay at a clearance sale
• Business that might have been wanted by three blind mice
• Business for blind mice as customers
• To get involved with it, real business may be done
• Outlet type
• Stores, markets, etc.
• Store-to-consumer business
• The trade serving the public
• That's about the last sort of business you'd have