Swapping quickly escalates: A little trade with a neighbor starts out innocent, but it's a slippery slope that can even lead to issues of embezzlement.
Trading decreases employee morale: The employees see the owner reap all the personal benefits of the trade, while the business as a whole gets nothing.
Employees often do what you do: When employees see the owner or other family members doing trades, they often take up the practice themselves with your business.
You devalue your offerings: When you trade, the value of the item or service (for both parties) diminishes, because no money is being exchanged.
You can't get equal service: Because you're getting the product or service for "free," it's difficult to complain when something isn't quite right.
