yeah, they made a traversing remote-control MG on MYTHBUSTERS like the one on BREAKING BAD........if I remember correctly, they use a garage door opener motor, welded some pieces for to make a platform, hooked up the motor to a battery and it worked. and they didn't pay no 600 bucks to make it.
that one on the auction site would be even simpler to make, and probably would cost less than a tank of gas. you just need a solenoid and a long, long cord to activate the trigger bar that is attached to the solenoid or some other mechanism . then just weld up prices of metal to make a platform