Is there somwthing about Apple’s mp3 players that will not allow me to play mp3’s downloaded from p2p sites?
yes, the RIAA.
outside of this, I don’t see anything which could stop you to use your player.
A couple years ago my friend had an apple mp3 player and he said that he had to get his musie from apple, because the player only played legally downloaded music.
Can mp3 players tell how I got the files and only play the legal ones?
I don’t think so ! All the Mp3 players I tied play all the music I load on it! I have heard some people download music of P2P and it plays on there Mp3
No issues with that on an iPod - except the legality of it, depending on your country of residence.
I think my friend has some software to bypass this, unless he uses the software for someonthing else,