Amazon has a coupon for 10 percent off the Topping D10b, if you use a store credit card for 5 percent more it's pretty good. Buying used is hard because they're so good, people only sell them to get something with more features or connectivity.
Topping DX3Pro+ for $149 in the used box down on the right.
Rtings has a review unit sale page where they sell things after checking them out. The cool thing is if you buy headphones there you were going to get anyway, you get to see all the test results and measurements they did of your exact pair.
www.rtings.com
Also Adorama is now on the Capital One credit card list for 4 percent cash back. It's a trick to use, go to Adorama and put what you want in a shopping cart, then log into Capitol One account, then look on the discount list and click through the Adorama link and buy from there to get the cash back on your card. With any other cash back on your card and Adorama points (which aren't worth that much but it helps) you can whittle the price down if you start at their
email list special pricing which is much lower than their advertised sky high prices. Sadly, eBay dropped off the discount program at the same time.