Buying music for iphone I usually don't buy content from Apple. I bought some iBooks, only to later find out that these can't be opened on other platforms. 

Now, I want to buy some music, and I wonder where I should buy it from. I may buy from iTunes, if it allows me to transfer and play songs on other platforms at a future date.

I usually just buy content from Amazon, at least with Kindle, they allow you to enjoy your content across various platforms. So, I also wanted to know if it is possible to buy songs from Amazon and play them on iPhone from iPhone's music app. I downloaded Amazon's music app, but it does not have any songs. I know that's weird, but the search didn't show any of the songs I searched for, and they were all popular songs.

So, really, given that I want to platform-proof my musical content, and yet play it on the iphone, where it should I purchase it from?
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.