Credit Card Payments.

When a merchant accepts payment on your credit card the Credit Card Bank will charge up to 4% of the total purchase to the merchant. Large retail stores pay considerably less due to their huge volume. If the product is returned and credited to the credit card, the merchant pays an additional 4% for the reverse transaction. This is why many retailers will offer a "store credit" if you return merchandise.

Audiophile stores will use your card as security but not actually run the total until you have evaluated the product in your system and want to buy it. That way, they avoid the double merchant charges by the credit card banks if the product is returned.