Art of Bartering
The general rule of thumb is that in stores you are not expected to barter, the prices are set. At the flea market or on the cuale, or on the beach, bartering is expected.
And yes, the general rule of thumb I follow is either to offer half the price quoted and start there, or I will sometimes cheat and ask my beach meseros (who are old friends) what the going rate is on something on the beach lately before I start the bartering process...just so I can get an idea of the retail markup. LOL