@ signora bianca if you open your box @ the store, and you find its defective, you should demand immediate replacement from that store's stock. generally the APR & AAR managers will allow it. if they refuse, you should contact AppleCare at 1800 803 638 and get them to sort it out.

failing that, you should demand a refund.

legally, if the product is defective ( i.e. not working / damaged ) you are entitled to a full refund of the purchase price.

if u buy from outside reseller if got many stock u can check everything n ask for exchange if got any defect on the spot..

IMO,
its always cheaper to get it online here but here's the catch, not every seller will have an extra unit waiting for u to exchange. some sellers will even ask for full payment before u open the seal box....
always get from more reputable sellers if possible and also knowing the origin of the mac would be an advantage before u proceed with the purchase.

they will surely let you check, unless they are scamming you.
if they refuse, the dont close the deal with them

I like the shop in Tropicana City Mall. Cant remember the name. Ground floor.