Well, as said, Buster is the name of a trigger, so "list Buster" returns the list of units present in Buster's trigger's area.
you may change Buster by this, so it refers to current trigger instead of a particular one.
Note that target variable is global, so if you have three triggers checking for the same type of condition and with the same on Activation code, there may be interferences if two of them activate at the very same time.
If you dont want to mess up with globals, you may just create a simple script this way:
;weirdscript.sqs
_trigger = _this select 0
_list = list _trigger
? count _list == 0:exit
_target = list _trigger select 0
_wp = EGrp6 AddWaypoint [getPos _target, 3]
exit
in the activation code of the triggers [this]exec"weirdscript.sqs"