Ebay! Set a good lower limit and see what happens. I've sold stuff on there for more than you could buy it on Amazon! If people get caught up in a spiralling bidding war, you could be in luck. It is a bit of a gamble though. You can get some great deals on there too. I got a 9.7 iPad pro, apple pencil, bluetooth keyboard and stand for 300 gbp. And in perfect condition!
Have you looked at swappa to sell it on?
Sold my 1st generation 12.9 locally today using Kijiji.ca Kijiji is similar to Craigslist but popular here in Canada. Did have to put up with a bunch of low ball offers and the odd person trying to waste my time but it sold in 3 days so not too bad. I always meet the buyer in a coffee shop for safety reasons and sales are cash only. This is how I’ve generally sold all my tech.