Buying music for iphone
iTunes is surely the most complete music online catalog for buying songs. Musics purchased from iTunes are linked to your iTunes account and can be re-download as long as you have access to your account on authorized device (iDevices or computers). Moreover from iTunes on a PC or Mac you can create an aac version of your track that remove your email address from the metadata of the track, the songs created this way can be transfered to other aac compatible devices.

One of the iTunes drawbacks is song quality : max 256kb/s, it's good for most people but not audiophile-proof.