Is it any iPhone App that I can use to buy / sell / transfer easily Bitcoin? I want to be able to download from Apple's App Store.
You cannot buy Bitcoin with credit card as far as I know.
iOS apps are generally limited to either:
- HTML5 client (Blockchain.info or GreenAddress.it)
- a "pure" (ie no website integration) app like BreadWallet, Pheeva
- A trading app (ANX), Coinbase
So buying BTC and managing or trading are very distinct functionalities, and you'll likely never find the former because of credit card fraud allowing for untraceable Bitcoins to be purchased. Also, most all purchases of Bitcoin (except OTC or cash LocalBitcoins.com transactions) require ID to allow fiat to BTC conversion.
The only exception I can think of is Coinjar.io which are rolling out a debit card in Australia which can be used like a normal credit card, but automatically debits a linked Coinjar BTC deposit.