Farfetch sells new items (from a variety of other store partners),
FashionPhile sells used items. I had a great experience with Farfetch (and things occasionally do go on sale there), but ymmv - I've seen some complaints about them being very difficult to return items to.
It is possible to get prices lower than the official Saint Laurent retail prices, at least for some of the most popular bags, but you may need to be very very patient and stalk the bags for a while. I price watched the Lou camera bag for about 6 months to get a sense of which sites it went on sale on and how low the price could drop before I finally pulled the trigger and bought it from Farfetch at a significant discount (and I was able to stack their student discount on top of it, and possibly also Rakuten cashback? I don't remember) - I managed to get it for 33% off the retail price. November seems to be the best time to buy bags in the US - cash back services like Rakuten seem to really ramp up their deals for the entire month before Black Friday (like, normally they'll offer 1-3% cash back, and then on random days in November it will be 10-15%), and that can be combined with whatever promo the store has going on.