The trade-in price history of iPad Pro 12 9 2018 WIFI Plus 4G 64GB generally shows a decline. In other words, the used prices of this device are dropping as newer models come to market. It is only very rarely that a second-hand electrical device will increase in price; this usually happens for example if it's a collector's item. Therefore if you are looking to sell your iPad Pro 12 9 2018 WIFI Plus 4G 64GB then it probably makes sense to do it sooner rather than later - before the next price drop.
Related Apple devices
Not sure if this is the correct device? Here are some similar ones that may also be of interest: