IOS – samples on Garageband on the iPhone

Actually, you can't. The linked article specifies exactly that you can't:

to create your own original music compositions or audio projects. You may broadcast and/or distribute your own music compositions or audio projects that were created using the Audio Content, however, individual audio loops may not be commercially or otherwise distributed on a standalone basis, nor may they be repackaged in whole or in part as audio samples, sound effects or music beds."

(emphasis mine)

You can create as many songs as you want using all the sounds you have in the loops and sell the songs. But you will be packaging the sounds (or repackaging) as audio samples of your application which seems to be forbidden.

Usually, we can easily import MP3 files to GarageBand (especially GarageBand 11)

However, sometimes the import process will encounter difficulty in importing an MP3 into GarageBand in two situations:

Situation 1: GarageBand only imports MP3 files which have the extension in lower case: ".mp3". If you try to import MP3 files (like MP3 recorded from the Olympus LS-10) with the extension in upper case: ".MP3", GarageBand will refuse to import those kinds of MP3 audio.

In this case, change the MP3 file extension to lower case in order to import MP3 to GarageBand.

Situation 2: GarageBand only supports those MP3 audio files which have a sample rate of 44.1 kilohertz (kHz) and a bit depth of 16 bits.

In this case, it will be necessary to convert the MP3 file(s) to GarageBand's more compatible AIFF file format.

The following link provides instructions for two methods to convert an MP3 file to a AIFF file at: How to Convert and Import MP3 to GarageBand?