bargain/negotiate/haggle

This is our first trip to Spain.... is it
acceptable to bargain/negotiate/haggle on
prices???? where and when is it done???? I hope to buy a guitar while in Spain and who knows what else may be discovered as a treasure.....

In small shops, the best way to get a discount is to say, "me da un descuento si pago al contado"? Will you give me a discount if I pay cash? They often will, since they have to pay a % to the credit company. Or, just ask nicely, hay un descuento? SMALL SHOPS ONLY! this is not done in big stores!