Sorry to post this again; God knows it's been covered but I can't find the thread anywhere, especially since the search function on this site pretty much sucks.
I have bought a CD changer off Ebay, and bought the data cord separately. I have an E36. I plugged everything up, only to find that the HU reads "Disk Error" on disk #4. Found some threads on that - tried things, banging it...etc. lol, to no avail. It won't eject properly.

I can do the famous el presidente trick to get the magazine out, but from the time i press the eject button and on, it makes this "humming" sound like some gears are rotating. I opened up the side where the lazer actually is, and I find that the white round knobs are turning, but nothing else will move. The tray won't slide up and down like I assume it is supposed to, and it won't go fetch any CD's nor attempt to play them.

used
Now I am wondering...the guys swears it worked before he sent it to me.

All I want to know is if something could've come loose during shipping or if it's a piece of junk. How can i fix this problem myself, or should I completely buy a new one or should I take it to a dealer and try to have them repair it? How much will that run me?
Mind you, I want to keep everything OEM
Thanks in advance guys.