The amount of markdown you can secure when buying used audio gear, like in other markets, depends on various factors including competition and current market conditions. For example, before COVID-19, it was easier to get a 20%-25% markdown below MSRP when buying a car in a buyer's market. However, the situation has changed significantly now.
When it comes to audio gear, in my area of West Palm Beach, there's only one dealer carrying high-end gear, and his prices for new items are fairly firm. However, he's more generous with discounts on used gear, which helps on his cash flow. Negotiating more than a 10% discount on new items is difficult, and it largely depends on the brands.