It's pretty rare to see a Lindsay for sale other than from Steve.
I think it's nothing to do with the systems themselves (I'm one of the ones fortunate enough to have both so I can tell you the differences are mostly a matter of preference).
But they're marketed differently.
GRS is big enough to have their lovely and seductive classes where you can buy your gear at a nice discount. As a result, many more people start off with GRS. Almost everyone who buys a Lindsay does it after having been engraving for a while. As a result, a few of the people who buy GRS later find out they have enough of a preference for the Lindsay system that they switch and sell off their GRS. There are more people who go to a class, buy a setup, and then later decide that either they don't really want to stick with engraving or have something come up that pushes them out. This puts more GRS systems on the used market.
Still, every now and then a Lindsay turns up so it is worth asking.
The other thing is that this gear holds its value pretty well so you don't always see that much of a savings buying used.