Welcome to the forum mate. As with most things, supply n demand plays a big role in this situation. As u would probably realise, brands that r high in demand like Rolex, Patek, AP etc. are going to be hard to come by at the AD. Discounts, if any, will be abysmally small. They really have little to no incentive to offer discounts when there r plenty of other buyers willing to premium.
Having said that, once we step out of the stuffy hype/viral bracket, theres a lot more room to breathe. Most other brands do offer discounts, and u r well within ur rights to expect it. Depending on the brand, we might even be offered 25% off straight from the get go. I'd say just play it cool, ask questions, don't settle for something if ur not satisfied, n be prepared to just walkaway if comes to that. Happy hunting.