GBA cable link
The GBA universal link cable is the only way to link two GBA's together, sadly.
Yes.
eBay
you can't a GBA link cable can't connect to a DS.
There is a opening in the link cable for another link cable to be inserted into that which then can transfer to the other GBA's.
yes
Yes. You must use the gba-gc link cable.
Use a GBA link cable.
The GBA universal link cable is the standard in linking GBA's together, and it'd be best not to substitute any other cable.
Yes but you also can trade using a GBA to GBA link cable.
http://www.amazon.com/Game-Boy-Advance-Link-Cable/dp/B00005BOS7