Supply and demand ... when they are competitively priced then they are not sticking around. Keep in mind that there is about 1 GS for every GT on the 2nd hand market currently. And not everyone who wants a Maserati wants a car as big as the GT or the QP.
Also, if a GS is absolutely immaculate then people will pay a little bit over the odds - I did! You can never see it in pictures, but some cars with 25K on the clock are pristine, whilst others are decidedly scruffy. This all impacts price, but is very hard to see from the online advert (especially when every seller calls their car 'immaculate'!).
So, this one is overpriced I agree by at least 3K, but maybe that is because it's a real minter and a lovely colour combo.