Instead of always saying "this site" and "on a site," try naming specifics.
The "other site" you refer to is Force FX. They have been sold out for over 6 months, because Jedi Arms Dealer (the one who makes couplers) has basically stopped. He did make couplers for the Mace Windu, Vader ESB, Anakin ROTS, and Luke ROTJ at one point in time.
The problem with a coupler for a standard FX, is that the pommels on them only have 2-3 threads. That means it can strip out VERY EASY. The Maul saber, on the other hand, as a good 8 threads, making it FAR harder to strip it.
To my knowledge, Jedi Arms Dealer didn't have the greatest quality control, and if a product was bad, he wouldn't replace it.