Will these be work in worldwide/uk ?
Its based on the shop you chose. Let me know on which gift card you are interested in.The one that i have Bestbuy.com and it cannot be used in .ca site.
i need itunes daily bulk
You rate doesn't seem to match with mine and i cannot provide bulk amount of codes. You can see my PM on my rate.
when we buy, do we have to use in a "certain amount of time" or can we keep, shop around, and buy feeling safe 
Its depends on the site gift card you have chosen. Most of the site provide long validity for their gift card. If you are asking about its genuineness then ofc it is. You can keep and use as per your wish until the gift card getting expired.